YT YouTube2Text

YouTube2Text → Transcripts → 08 -- Energy -- Sweet Conceptual Physics By Paul Hewitt

08 -- Energy -- Sweet Conceptual Physics By Paul Hewitt — Transcript

by LearnHub · 7,406 words · 1,073 segments · language en · Watch on YouTube

Open in transcriber Summarize with AI Download SRT/VTT

Full transcript

0:00e
0:04[Music]
0:05[Applause]
0:09[Music]
0:12[Applause]
0:20[Music]
0:22[Applause]
0:22[Music]
0:28[Applause]
0:29[Music]
0:32[Applause]
0:37[Music]
0:50what chapter are we going to talk about
0:51today
0:53gang what's it what is it begins with e
0:56energy how many people feel energy how
0:59many people wish they
1:00more we'll be talking about such thing
1:02now but before we talk about energy I
1:04want to talk about something that was
1:05somewhat related uh we talked before
1:08about the idea of momentum remember
1:11momentum momentum is inertia and motion
1:14and how do you get something moving if
1:17it's not moving how do you change the
1:19momentum of something you push on it and
1:22you provide what's called an Impulse
1:24impulse right impulse is not only
1:27Force it's not how hard you push only
1:30that will tell you how much momentum
1:31change you'll get it's how long you push
1:34and we talked about how long in terms of
1:36time didn't we we said Force multiplied
1:39by time would be numerically equal to
1:42you multiply the force that you exert on
1:44something huh multiply by the time that
1:47you exert on something you'll get a
1:48numerical quantity and that quantity is
1:50exactly equal to not the momentum but
1:53the change in momentum that you you
1:56produce by pushing
1:58huh and so we talked about this you know
2:01and we called this idea here how hard
2:03you push multiply by the duration in
2:05which pushing takes place we had a name
2:07for that see if your neighbor remembers
2:11the
2:14name don't be an imp now come on come
2:17on what is it gang impulse yeah don't be
2:21afraid to be a little verbal all
2:25right and so we said impulse is a change
2:28in the quantity and inertia and motion
2:31moving inertia we call that what that
2:33what do we call it begin with an M
2:37momentum okay don't be
2:42shy and that's what we talked about
2:44right and we said for big changes in
2:47momentum you'll have what kind of
2:49impulses begin with B big for small
2:52changes in momentum you'll have what
2:54kind of impulse begin with S small all
2:57right now what happens when what happens
2:59to the change in momentum when bouncing
3:02occurs larger or the same as if bouncing
3:05does not occur begin with a l larger and
3:09we can show that with this device here
3:11here's a
3:12dart it's like a dart and I have a sharp
3:15nail in there and what I'm going to do
3:17is I'm going to lift the dart up and I'm
3:19going to allow it to swing against that
3:21block it's going to make a collision
3:22watch
3:24this the block did not tip I'll try it
3:28again
3:32the block did not tip this time I'm
3:35going to remove the point and now I have
3:39just rubber here this time when I swing
3:42it it's going to bounce and when it
3:45bounces more impact or less
3:49more wow hands down no no contest I
3:53don't even have to lift it even as high
3:55I can even lift it this High
3:56watch hey so bouncing
4:00more change in momentum when it bounces
4:02when this come down and stop boom
4:04there's a certain change but when it
4:06hits stops and then it comes back out
4:08again then what more change or less more
4:12more change more change in momentum
4:14means what kind of impulse greater
4:16impulse and you saw that and it's the
4:18impulse that knocked the block over hey
4:20huh huh
4:23physics we're not going to talk about
4:25impulse and momentum today we're going
4:27to talk about a related idea how hard
4:30you push to get something going is one
4:32thing but how long you push is also
4:34important so far we talked about how
4:37long in terms of time how long in terms
4:41of what your watch reads we can talk
4:43about how long in terms
4:46of distance that's right and when we do
4:49that force times distance it turns out
4:53we'll get a change in some other
4:54quantity and that other quantity is not
4:56momentum it is begins with the e
5:00okay and this is what we're going to
5:02talk about
5:05today force times distance will produce
5:08a change in
5:10energy what is
5:12energy energy is that numerical quantity
5:16which appears in many many forms like
5:19mechanical energy we'll talk about
5:21mechanical energy today really energy of
5:23position potential energy energy of
5:25motion motion energy we don't say motion
5:28energy we use Greek word for motion
5:30begins with a K check your neighbor see
5:32if your neighbors on what kind of what's
5:35what what's motion energy with K kinetic
5:37yeah kinetic energy okay and then we
5:40could talk about later on we can talk
5:41about heat
5:43energy sound energy can anyone think of
5:47any other kinds of energy how many say
5:49no that's probably it there's probably
5:50no more come on gang some more light
5:58energy good radiant energy radium from
6:00radium things like that huh theral good
6:03thermal
6:04energy good nuclear energy and turns out
6:07energy of being it turns out everything
6:11has this quantity called Energy and
6:13that's given by Einstein celebrated
6:20equation later in the course we'll talk
6:22more about this the idea that energy and
6:24mass are two sides gang of the same coin
6:28you guys get Mass right you know what
6:30your mass is squashed up energy you are
6:33all bits of energy all squashed up and
6:36the more massive you are the more energy
6:38you have and sometimes you can convert
6:40from the inertia kind to the radiant
6:42kind and that's what happens in the sun
6:43and the stars and that's the energy of
6:45fusion and we'll talk about that too but
6:48today we're only going to talk about the
6:50simplest kind of energy mechanical
6:52energy and it's related idea work
6:56because this force times distance we
6:57don't call that impulse gang no Force
7:00time time is impulse we call Force time
7:03distance another word see if your
7:05neighbor doesn't know what it
7:09is who came
7:12unprepared doesn't
7:16know what is it gang all together work
7:19all right work we call that
7:22work okay work equals a change in and
7:26I'll just put e for energy all right
7:29if I'm lifting something up if I lift
7:33this block up I do work on the Block
7:34when I lift it because I'm exerting a
7:37force through some distance okay and the
7:40higher I lift it the more work I do the
7:44more work I do the more what it has
7:46begins with the E energy okay if I
7:49Elevate way way way up it's got a lot of
7:52energy and we call that energy energy of
7:55position and that energy of position you
7:57could you could put the work in today
7:58and you could use it tomorrow so you can
8:01store it it can be there for your
8:03potential use so that being the case
8:06gang we got a name for energy of
8:08position begin with PE what is it
8:12potential energy potential energy is
8:14energy of position we talked before
8:16about a slingshot when you pull a
8:18slingshot back further okay Boom the
8:20projectile is going to go faster and we
8:22said it going to go faster get more
8:24momentum today we're going to say yeah
8:26let's get more momentum but if it has
8:28momentum it also has something else
8:30begins with a e try it all right ends
8:34with J ends with
8:37Y energy okay but now it's energy of
8:41motion and we call Kinetic we
8:44call energy of motion what gang yeah so
8:48I can go like this and pull it with
8:49position more position more position
8:52more energy more energy whip boom more
8:55what more kinetic energy that's right
8:58how about cannons how about you get a
9:00cannon like
9:01this it's that long okay and you fire a
9:05cannonball out come the cannon ball huh
9:07let's suppose you have another Cannon
9:09same kind of Cannon only
9:12larger now you fire the same kind of
9:14cannon ball the same amount of powder
9:17out it goes in which case will the
9:19canonball go
9:21further how many say the longer one show
9:23a hands e how many say the longer one
9:26because there's more impulse on it
9:30show her
9:31hands how many say wait a minute impul
9:34impulse was the other day I don't know
9:36about
9:37today come on how impulse what is
9:39impulse gang it's a force than the time
9:42during which it acts but couldn't we
9:45also say hey it's going to go further
9:47because there's more energy given to it
9:50how many see
9:51that even if the force is the same cuz
9:55over here I hit with a certain Force
9:57here a certain Force but here the Force
9:59acts over this
10:01distance here the force acts over this
10:03distance which is bigger this quantity
10:06or this quantity this quantity so you're
10:09get you're doing more work on the
10:10Cannonball so it's going to leave with
10:12more kinetic energy and we can kind of
10:14see if something got a lot of kinetic
10:15energy I got a lot of momentum if I got
10:17a little kinetic energy I got a little
10:18momentum they're
10:20related they're related that means it
10:22can be confusing sometimes we're not
10:24going to push that to death though just
10:26want you to know that when you do work
10:27on something you can can change the
10:30energy that it
10:34has I can kind of show you that with
10:36this bowling ball over
10:39here here's a bowling ball suspender
10:42from the ceiling and what I'm going to
10:44do is I'm going to do work on this
10:45bowling ball and increase its potential
10:47energy cuz what I'm going to do is I'm
10:49going to pull the bowling ball back now
10:51it's going to take a force to do that
10:53this little device here will show the
10:55force that I apply and we can look at
10:58this kind of close up and we can see
11:00that as I pull
11:01it more and more the force gets more
11:04greater greater greater notice the force
11:07is varying with the angle it gets very
11:10complicated to multiply the force times
11:13the distance because the force keeps
11:15changing with the distance so to
11:17calculate that you need a kind of
11:19mathematics that we're not into right
11:21now it's called calculus but it turns
11:23out when I do that and I pull with an
11:25increasing increasing increasing Force
11:27higher higher higher higher higher I'll
11:29get a particular number here but you
11:31know what I've raised it this high off
11:34the ground if I simply pick it up
11:37straight up it takes more Force but I
11:40lose I I I go a smaller distance and
11:43guess what that greater force times a
11:44smaller distance is compared to the
11:46variable force times a long distance
11:49take one guess what the energy is at the
11:51end both ways same take a guess begins
11:55with a SS now take a
11:57guess how many sit next to someone oh
12:00check check the neighbor check the
12:10neighbor what is it gang same same
12:13that's right that's right I'll show you
12:15another nice thing with this
12:18pendulum this pendulum illustrates one
12:21of the most important ideas in
12:22physics what I'm going to do is I'm
12:24going to hold this against my teeth and
12:26I'm going to let it go okay and you and
12:29see what
12:31happens fall short I didn't Flinch man
12:35should I do it again I'll close my
12:41eyes I I really did have my eyes closed
12:43okay you know what gang what's this
12:47illustrating how many say oh it's
12:49probably not illustrating anything it's
12:51just a bowling ball going back and come
12:53on you guys this illustrates energy
12:57conservation the idea that you don't
12:59don't create or you don't destroy energy
13:01you just slish slos it from one form to
13:04another when I lifted this up it had
13:06energy of what begin with p position
13:09right and I let it go it turns into
13:11energy of motion kinetic right and so
13:14the position motion position motion
13:16position but here's the idea when I
13:19started I did work and lifted it to a
13:21certain amount it's got a certain amount
13:22of energy call it a th units when I let
13:25it
13:26go whoops okay well I did a little work
13:29on the wall okay but the idea is it will
13:31never come up higher if it came up
13:34higher it would have more than a th000
13:36units and we have a little dictum in
13:38physics no can be or we've never seen
13:43such a thing never as anyone recorded
13:46and verified the idea that something
13:49could put out more energy than was put
13:51into
13:51it we call that idea the conservation of
13:55energy
14:03the
14:05pendulum at this level is lifted and has
14:08a certain potential energy at this point
14:11a PO potential energy with respect to
14:14say this level that's how much potential
14:16energy maybe it's a th units let's call
14:18it a
14:20th those units by the way would be
14:22called Jewels Newton meter we're not
14:25going to get too much into the into that
14:27too deeply into that but say thousand
14:29units of energy here now it goes into no
14:32more potential energy it's all what kind
14:34of energy gang
14:36kinetic some people know the value of
14:39the kinetic energy there and some people
14:41don't see if you're sitting next to
14:43someone who
14:52[Music]
14:54do how many say it's a
14:57thousand how many don't really know how
14:59I don't know I haven't paying attention
15:00man I don't know what it is show a
15:04hands it's a thousand yeah a th000
15:06jewels and when it goes to the other
15:08side what will it be a thousand or not
15:12quite it turns out not quite boom
15:16Energy's been destroyed yes or no answer
15:19begin with an
15:20N account for the energy then it turns
15:24out when you're swinging it never quite
15:26comes back to where it was you know why
15:29it's transferring the energy somewhere
15:32else guess
15:36where how's the temperature feel today
15:39gang relatively
15:43cool how many say that's strange it
15:46seems to be warming
15:48up okay A little exaggeration but it is
15:52that thing's banging into the air and
15:54when it bangs into the air boom what
15:55happened to the air molecule move a
15:56little faster and we'll learn later on
15:58that itic energy of air molecules honey
16:00we talking about
16:02temperature and so what'll happen is the
16:04energy go from that to the energy of the
16:05molecules in the room it dissipates in
16:07the form of heat see but you always have
16:11it there it always adds up and that's
16:14kind of nice all the physics experiments
16:16you do you always find out the energy
16:18score Remains the Same and sometimes
16:20when it doesn't seem to you discover
16:22things that maybe you didn't know about
16:23before it's marvelous a marvelous
16:26principle of physics conservation of
16:28energy let calculate the energy of any
16:31system maybe it's a Galaxy about to
16:32explode calculate the total energy let
16:35that Galaxy do no matter what it wants
16:37to do is spin around I don't care
16:39what some some big event then draw a
16:42dotted line around it calculate the
16:44energy in there again what do you got
16:45guess what same darn number it might be
16:48different forms now maybe more light
16:49than before but the energy just
16:51transforms from one form to another and
16:53that's kind of
16:57neat let's suppose we have the case of a
17:00person up at the top of a flag
17:04bow and they're up at the top of that
17:06flag bow and they're going to dive into
17:08a little bucket of water he used to do
17:11this in circuses okay and here's a dude
17:13up here he's going to jump and let's
17:16suppose up at the top there he has
17:1910,000 units of energy that's his
17:22potential energy isn't it he's at rest
17:25so being at rest what would his kinetic
17:27energy be zero zero
17:31would just say zero
17:35and right into the bucket when he gets
17:37down to the bottom of the bucket the
17:39potential energy is what zero zero cuz
17:42we took this with respect to the ground
17:45so down here it's got a potential energy
17:46of zero I know you're sitting next to
17:48someone knows what the kinetic energy of
17:50splat is I know you do that just just
17:53double check to be sure what's the value
17:55of the kinetic energy
17:57here how much kinetic energy 10,000 how
18:00many people don't say
18:0310,000 come on you guys there's going to
18:05be
18:0610,000 10,000 units hey when the dude
18:09gets halfway down the potential energy
18:12is only
18:14half 5,000 there are people can
18:18calculate what the kinetic energy must
18:20be see if you're sitting next to one
18:29what's it going to be
18:30g000 5,000 let's suppose when he gets
18:333/4 of the way down his potential energy
18:35over here is now only what one quarter
18:38of this one quarter of that looks like
18:422500 will the kinetic energy be more or
18:45less you know it's going to be more
18:47picking up huh and what will the kinetic
18:49energy be at the at the one quarter up
18:51Mount huh
18:5475 such that what did you know do you
18:57see the rule here here here here here
19:00here anywhere going to have the same
19:03energy and when he finally comes down
19:05here the kinetic energy goes to zero
19:06boom calculate the heat of the bucket
19:08and the heat of his remains and what do
19:10you got then you've got 10,000 units of
19:12heat energy and that dissipates
19:14dissipates dissipates but it's always
19:16there energy score stays the
19:25same show you something kind of neat
19:33here I've got
19:36a piece of metal I'm going to weigh
19:46it Hunker metal weighs a little bit more
19:48than 10 Newtons just about 10 and 1 12
19:51Newtons see that okay 10 and 1 12
19:54Newtons now what I'm going to do is I'm
19:56going to hold it with two scales
19:59and you guys tell me what's the reading
20:01going to be in each scale now this is
20:04old stuff one scale there one scale here
20:09wish I had one more
20:15hand it should be half gang is it half
20:20let me take this one
20:22off and at an angle it gets wild
20:26information overload today so we'll only
20:29talk about straight up and down
20:42okay my clutchness is showing there we
20:46go half so I can hold it up with half as
20:49much force when this supplies the other
20:53side huh is that right okay now I'm
20:56going to do work on this and lift it
20:58okay
21:01let's suppose first of all I do work and
21:03I lift it like
21:05this I want to lift it let's suppose I
21:09want to lift that up to
21:10the 20 cm Mark okay right to here okay
21:14watch
21:17this how high did I move my hand I move
21:20the bottom of this up 20
21:22cm how far did my hand move more less or
21:26the
21:27same did you see it's the
21:29same if you take that reading which is
21:32about 10 and2 and multiply it by the 20
21:35cm you'll get a number okay that's a
21:39number that's the amount of work I'm
21:41doing when I lift it up okay and it's
21:45going to take that amount of work to
21:47lift it up 20 cm let me put that on the
21:49board
21:59the work I'm going to do is force times
22:02distance is going to be it would let's
22:04round it off to 10 okay 10 Newtons and
22:08the distance I raised it where it was 20
22:10cm or 210 10 of a meter 2/10 of a meter
22:14and that's going to be equal to two
22:17Newton meter and we call that a jewel
22:20two jewels of work to lift it up 20
22:24cm I'm going to lift it up 20 cm again
22:28but I don't like having to pull so hard
22:30I don't want to pull with 10 Newtons I'm
22:32getting tired I'd like to pull with
22:34about five Newtons how could I do
22:37that I could tie this onto
22:41here we have to wonder if the Ancients
22:44who constructed the pyramids knew about
22:46so simple an idea as we're talking about
22:49today because now I can lift it as
22:52you've seen with half half the
22:55force half the force now I'm going to
22:58lift this
22:59up 20 cm and you watch how far my hand
23:03moves where's the 20 cm Mark right
23:06[Applause]
23:07here how far did my hand move more the
23:10same or less much more can you see twice
23:13as much I moved my hand through 4/10 of
23:16a meter but the force I exerted was only
23:205 Newtons but I had to exert it through
23:23through twice as much and what's the
23:25result gang two jewels
23:29this is like a
23:30machine some people think that a machine
23:33can change the amount of energy those
23:36people are wrong a machine can't put out
23:40any more energy any more work than put
23:43into it what a machine can do is change
23:47the force that's required to do that
23:51work so whether it's a pulley system or
23:54whether it's a lever the work you put in
23:57is the work you get get out and you
23:59could hold the lever at different
24:01positions and you could get different
24:03forces to do the same work but watch out
24:06gang no machine can put out more energy
24:10than has put into it okay what can it
24:14change see if your neighbor
24:22knows we can kind of see the answer to
24:24that when we look at the equation for
24:26work work is what force times distance
24:29since the work input work output is the
24:31same what you can do is you can slish
24:33slash back and forth between whether
24:35you're increasing force or increasing
24:37distance consider a huge truck with a
24:40flat tire and that truck got to be
24:43lifted so the tire can be taken off can
24:46you imagine the Ancients back when the
24:48pyramids were being built seeing a
24:50little girl using a bumper
24:52jack to lift the Mac Truck a little
24:56kid and that come that say they see the
24:59kid go they see the truck come up in the
25:01air my gosh kid how did you how did you
25:04lift that truck and she said well I had
25:07to just do some work on the Jack and the
25:09work on the jack did work in the truck
25:11and the work on the truck same as the
25:12work on the jack no big deal this is
25:1420th century stuff he says but I'm not
25:1720th century yet could you explain that
25:18to me and the little girl would say this
25:21well look at this when I push
25:23down she say I have to push only a
25:25little force compared to the force of
25:27the truck
25:29but when I push down look how far I got
25:31to
25:32go she said multiply my push times the
25:35distance I push you get a number right
25:37and all of people say yeah you got a
25:39number now multiply the force on the
25:41other side by how high it go and you get
25:44a number right and guess how big the
25:46numbers are compared to each other gang
25:48same
25:49same what if she went like this click
25:52click and the truck went click
25:55click honey that's magic that's not
25:58physic you hear me okay but isn't that
26:00neat that it always turns out to be just
26:03the force down time the distance equals
26:05a force times the distance we kind of
26:07show that like this the force she might
26:09push it be like that and the force she
26:11lifts might be like that but the
26:14distance she pushes is like that and the
26:17distance this Rises is like that and
26:20these would be the same not quite the
26:23same some of the energy goes into
26:25working and heating up the jack a little
26:26bit so this number is always a little
26:28bit smaller and how much smaller we get
26:31into the idea of efficiency won't talk
26:33about that today but with ideal
26:35efficiency this can never be bigger than
26:39this so you're walking down the street
26:41and you see some some dude pulling in a
26:43pulley system and you see him
26:45single-handedly lifting a piano and a
26:46piano is going up in the air a piano
26:48that a human being could never lift one
26:50of the grand grand piano type jobs okay
26:53and here we go ch ch ch ch ch ch and
26:55that piano's going up in the
26:57air how can be such a
27:00thing well first of all if the pulley
27:03system means that the piano is being
27:04held by 10
27:06ropes then the Rope tension will be the
27:08same in all 10 barring friction then his
27:11free end he's pulling one of those
27:13tensions the same as those so he's got
27:15one tenth the weight of the piano on
27:17every rope the one he's pulling so the
27:19piano weighs a th he pulls a tenth of a
27:21th what's that 100 if he can pull 100 up
27:24it goes but furthermore you multiply how
27:28many meters of rope go through his
27:31hands by the force he pulls you get a
27:33number yeah now multiply the weight of
27:36the piano Among Us H times the amount of
27:40meters it goes up smaller number yeah
27:42multiply those two together what's the
27:43biggest that number can
27:45be y same same a't that neat it works it
27:50works
27:57energy I remember years ago I used to
27:59hear people talk about the the so-called
28:01conspiracy of the oil
28:04companies that their uh cars automobiles
28:07with little gadgets usually involving
28:10magnets attached to the carburetors of
28:12the cars would allow cars to go
28:15fantastic distances but the oil
28:18companies have bought up the patents and
28:20they're withholding it from the public
28:21so the public will have to guzzle down
28:23more oil make more money for the
28:24companies see the idea that maybe I was
28:27told one time time for example that with
28:29these devices one could put a couple of
28:31liters of gasoline in your car and drive
28:34from Boston Massachusetts to San
28:35Francisco and a couple of laders but the
28:38oil companies won't let you do it so you
28:40have to burn a couple of barrels or
28:41something like that well how about that
28:44gang is there any is are there any rules
28:47that govern this sort of thing answer
28:49begin with a
28:51Why Try it ends with a p try it yep
28:56there is and that's the energy
28:58conservation and let's talk about that
29:00right
29:01here okay we're going to push the car
29:04across country it's going to take energy
29:07to do that it's going to take energy to
29:09do work on the car and that work is the
29:12force times the distance that we're
29:13pushing the car now what's the force
29:15that keeps a car going even in a
29:17straight road what's the car what's the
29:19force it's the force against friction
29:22and for a very streamlined car that
29:24force can be something like 1,000
29:26Newtons a th000 Newtons at highway speed
29:29so let's see what
29:31happens what's the energy over here
29:33that's the energy given to us by the
29:35gasoline and that's a certain amount it
29:38turns out gasoline has an energy content
29:41of about 40 million Jews per liter 40
29:46million now this is what the energy
29:50thing means if you have 40 million units
29:53of energy to spend
30:01that's the energy content in your
30:02gasoline what we're saying is these two
30:06numbers multiplied together can't be
30:08bigger than that cuz that's what you got
30:11to spend in your gas tank I don't care
30:12what kind of Gadget you got that's
30:16it now how you going to do it you're
30:18going to push your car through some
30:21distance let's see how far you can push
30:24it how far can you push it well if
30:26you're pushing it uphill it's going to
30:28take more Force so this will be smaller
30:30pushing a downhill maybe gravity will do
30:31it for you you don't need anything okay
30:33but along a level Road you got to push
30:34against the Air drag isn't that true the
30:36faster you go to my Air drag and let's
30:38take a an average unit of uh 1,000
30:41Newtons 1,000 Newtons for our force and
30:45now we want to calculate can we
30:46calculate gang at our seats huh pencil
30:49and paper what's the distance that the
30:51car could go with a th units units of
30:55force pushing it along how many kilm
30:58will the car go on that one liter
31:02calculate such thing
31:23now am I doing it right gang am I doing
31:26it right if this time this equals that
31:29then this equals that divided by this is
31:31that right that is that the algebra we
31:33can do that look at the zeros
31:36honey now anyone have a
31:40calculator how about without a
31:41calculator we can do huh one goes into
31:4440,000 how many times 40,000 so this car
31:47is going to go 40,000
31:49M and it turns out if we express force
31:53in Newtons distance in meters and energy
31:55in Jews the units will match so we have
31:57no verions to make so this turns out to
32:00be 40,000 M we don't say 40,000 M we say
32:0540
32:06kilomet so it turns out to be 40
32:11kilm you know what this means gang in a
32:14car that is 100% efficient you can't
32:17have that make believe you can 100%
32:21efficient that means all the energy of
32:23the gasoline goes into moving the car
32:26none goes into the heat none goes into
32:27the sound none goes into the friction
32:29huh all into the car it's the upper
32:32limit if you have to push with a th000
32:35Newtons which is reasonable then the
32:38greatest distance you can possibly get
32:40Nature's Nature's ultimate is 40 km so
32:44this car will give you 40 km per
32:47liter and there's only one way you can
32:49get more how's
32:51that what is that this is huh how can
32:54you get more distance you go outside the
32:56car and you push on it Yourself by
32:58changing the force that's right if you
33:00can decrease the force then you can
33:02increase the distance do you see that
33:04and how do we decrease the force with
33:07cars we make them streamlined so they
33:10cut through less Air drag as they move
33:12isn't that nice see and usually you're
33:15going to get about 25% efficiency which
33:18means you get 25% of this okay which
33:20means you usually get about 10 km per
33:22liter see yeah so what you're trying to
33:25do is to increase the efficiency but
33:27this is in the upper limit see now most
33:31people don't realize this people don't
33:34know there's an upper limit that
33:36Nature's limit and they think that
33:37little gadgets or something can how
33:39somehow beat this and many of the
33:42devices that are submitted to the patent
33:44office flout this all together and the
33:47first thing the patent types do with
33:49some particular machine they say wait a
33:50minute is it going to violate the
33:52conservation of energy if it does we
33:54don't want to look at it and the reason
33:56is the patent people have through
33:58thousands and thousands and thousands of
34:00schemes maybe hundreds and hundreds
34:01hundreds of schemes which all seem to
34:03give you out more energy than before and
34:05they find out where the little flaw is
34:07so at this point they just say look if
34:09it violates energy conservation come on
34:11it's another whacko wacko because
34:13nothing has so far shown that you can
34:16get more out than you put
34:19in so there are rules to the universe
34:22and that's one energy conservation you
34:25can change from one form to another but
34:27you can't get more than you started with
34:30no free lunch
34:32gang and with most machines you don't
34:35even break even because some of that
34:37energy goes into overcoming friction
34:39turns into
34:42heat so why do you eat your corn flakes
34:45in the morning
34:46gang you keep eat your corn flakes in
34:48the morning because what do you do those
34:50things are being torn apart in your
34:52stomach when it torn apart the bonds
34:53that energy becomes part of guess who
34:56and you use that to go about your day
34:58and what happens you stop eating how
35:00long can you keep running your car
35:02without putting gasoline in how long can
35:04you keep running your body engine
35:05without putting food in so the
35:07conservation of energy hey biology right
35:17underneath here's a neat little device
35:19here it's a
35:21generator consists of magnets these
35:25magnets and inside here I have a coil of
35:27wire and I'm going to turn that coil of
35:30wire and the coil of wire is going to
35:32turn inside the region called a magnetic
35:35field and when it does that it's going
35:37to induce electricity and when I do that
35:40I can light the
35:45lamp how that happens we'll talk about
35:48later in the course but for now we can
35:51look at this in terms of energy that
35:54light that you saw glowing that's a form
35:57of energy energ what was the source of
36:00that
36:01energy it was the Corn Flakes I ate this
36:04morning because it takes work I have to
36:07work on this to turn it see
36:10that and if I want to make it brighter I
36:13got to work more isn't that neat okay
36:17later we'll learn that if someone could
36:18keep turning this thing we could
36:20generate electricity to light up cities
36:22and sure enough we do and we have
36:24devices like this and we put a waterfall
36:26over here and turn it use the energy of
36:28the waterfall potential kinetic
36:31rotational mechanical off it goes as
36:33light huh do that or we could put a
36:35steam turbine here direct some expanding
36:37steam against a turbine against a Paddle
36:39Wheel and turn it and so our
36:41civilization really rests on devices
36:43like this but there's got to be some
36:45energy input to be transformed over here
36:49now here's a neat little thing when I
36:52turn this and I unscrew the lamp
36:59it becomes easier to turn easier to turn
37:03why well when it's
37:06unhooked there's no energy going off and
37:09so all I'm turning against now is the
37:11friction but when it's hooked up I'm
37:14pushing against the friction plus the
37:16electricity so let's I I can show you
37:18that can I have a volunteer could you
37:19come up here
37:22please what I'm going to do could you
37:24stand over here and what I want you to
37:26do is I'm not going to look I'm going
37:27going to look over here okay and I want
37:29you to unscrew that and I'll tell you
37:31when it's unscrewed cuz I'll feel the
37:33difference over here so let's tighten it
37:35up now okay I'm going to do it
37:40okay ah you just
37:42unscrewed screw it again ah now it's
37:46on ah
37:47off how do I know I'm looking as a
37:50mirror right up
37:52here no but you see that you try it you
37:55try it
38:04Can you feel it easier just do it go
38:06ahead see if it gets
38:08harder okay keep keep a nice steady
38:11Cadence
38:13yeah mind mirror okay okay here we go go
38:17ahead go
38:19ahead no how about now harder harder
38:24okay here gone
38:28is that nice huh huh huh okay okay the
38:32handle knows whether the lamp is
38:35on how about that what's going on there
38:42gang even why that happens the mechanics
38:45of why that happens we going to get up
38:47to and we get up to electricity and
38:49magnetism delicious stuff okay but we
38:53see that
38:54energy conserved the energy I put in the
38:57energy come out okay like that much
39:00easier to turn it try it after class
39:03yourself it takes energy to light the
39:07lamp that being the case let me ask you
39:11a
39:12question you get your car you want to
39:16find out what your gas mileage is cuz
39:17you want to compare it with your
39:18friend's cars you all got a contest this
39:21friend is bragging about getting so much
39:23mileage so much mileage you want to test
39:25yours too so you driving your car and
39:28you're driving along the highway you're
39:29looking at the speedometry you see how
39:30miles you go tank it up later on see how
39:32much gas divide one to the other you see
39:34how many miles per gallon you got how
39:35many kilometers per liter yeah and
39:38you're doing that and you're driving
39:39along and as you're doing that someone
39:41reaches over and turns on the
39:43radio and pulls out the light switch and
39:47pushes in the cigarette lighter and you
39:50say would you cut that out you're
39:52screwing up my gas
39:54mileage true or false
39:58check your
40:03neighbor what's the answer
40:05gang begin with the F end with alt does
40:10it begin with t ends with Rue which one
40:13how many say
40:15true how many say hey when you're
40:18driving at night time if you want to
40:19save on gas what should you do put your
40:22lights
40:24off people say how come you're driving
40:26with no lights I'm trying to save gas
40:28true or false true no true that's right
40:31so I want you guys to all learn
40:32something from this course I don't want
40:33you driving at night with your lights on
40:34drive with your lights off
40:36right no we maybe it's worth to burn a
40:39little more gasoline to keep from
40:40crunching up huh but nevertheless you
40:42are burning more gasoline when you put
40:45your lights on see if your friends know
40:47that now a lot or a little answer begins
40:49with an
40:50L probably a little okay probably a
40:53little but you really are there's no
40:54free lunch how about your airon in your
40:56car air conditioner when you put the air
40:58con on does it burn more gas something
41:01what an air con just just hooked at I
41:02mean a belt from it spins that thing
41:03that's just hooked down to the drive
41:05shaft anyway the old drive have turnning
41:07the hook on I mean that doesn't know the
41:09difference it doesn't know the
41:10difference did this know the difference
41:12when this was on it knows the difference
41:15it's got to push it's got to do work
41:16you're going to burn a lot more gas with
41:18your air conditioner on you know why
41:20especially on a hot hot day because now
41:23you're having to put out more energy to
41:25get to to to to to to uh air condition
41:28the car more energy how we do that we
41:31going to be talking
41:34about later okay talk about that too
41:37ain't that nice a whole lot of ideas we
41:39can talk about because all these ideas
41:40begin with f what are they physics
41:42that's right
41:44yeah let's talk a little bit about
41:47kinetic energy I haven't given you the
41:50relationship for kinetic energy yet let
41:52me give it to you
42:02it's a little
42:08tricky I'm not going to derive it it's
42:11derived in the footnote in your text
42:14I'll just state it without
42:18proof it's half the mass multiplied by
42:22the speed
42:24square remember momentum was just Mass
42:27time speed simple simple this somewhat
42:30more complex because it's half the mass
42:33time the speed
42:35square the square of the speed tells you
42:38that wow it really depends on speed
42:42because it's speed multiplied by speed
42:45so it's very very speed
42:48dependent when we talked about firing
42:51the rifle
42:53before the momentum of the bullet equal
42:57momentum of the rifle coming back so
43:00momentum momentum same impulse impulse
43:04same but that was kind of bothersome to
43:07some people cuz this has got something
43:09more than this and you know what it has
43:11more of energy okay because the had a
43:15bigger speed but now with energy it's
43:18squared woo so the Bullet going out
43:21might have the same momentum as the gun
43:22coming back but honey most of the energy
43:26is in the bullet fo not the gun cuz
43:29we're squaring that greater
43:32speed you square the speed the results
43:36of give you an
43:37example you're driving down the road in
43:40your car and you're going 30 kilomet per
43:43hour and you you step on the brakes you
43:47Screech to a stop you're going to change
43:50your kinetic energy you're going to
43:52change it from the kinetic energy you
43:53have at 30 km/ hour down to zero What's
43:56it take to change that kinetic energy
43:58gain it takes work and that work is what
44:02what kind of work stops your
44:04car we can let the equation guide our
44:07thinking it's a force and it's a
44:08distance distance is the distance your
44:10car skids yeah and the force must be the
44:13friction force of the wheels against the
44:15road yeah okay yeah I mean yeah between
44:18the wheels and the road okay so that's
44:19the friction Force that's the distance
44:22okay so you let's suppose you skid 10 m
44:2610 m
44:28now we repeat we go twice as fast we get
44:32twice the momentum yeah we go twice as
44:35fast we step on The Brak so we skid to a
44:38stop we skid
44:40further some people say well twice as
44:43fast you're going to Skid twice as far
44:46tilt not twice as far how much far check
44:51the neighbor see if the neighbor knows
44:52already
44:58twice as fast gang how many say twice as
45:01far how many say more than twice as
45:04far how many can calculate it to be f u
45:09r four times as much so you're going to
45:12SK if you got four times the energy
45:14you're going to
45:15Skid four times as far it turns out the
45:18friction Force won't change with speed
45:21how much the grab the tires in the road
45:23grab each other does not have to do with
45:24speed see so you get the same friction
45:27now the same friction is going to drag
45:29you four times as
45:31far that's why people go on a little bit
45:33when you first learn to drive you've got
45:35to pick this up Savvy wise you know you
45:37go a little bit faster you stim on the
45:39brakes and it's surprising at first that
45:40wow it took so much longer to stop or
45:43you Bo that car up to three times as
45:44fast now three times the speed okay so
45:47not 30 not 60 but 90 km per hour now
45:51with three times the speed you know you
45:53got more energy more kinetic energy some
45:56people know how much more check with a
46:02neighbor how many say nine times as much
46:06energy that means you're going to Skid
46:08how much further nine times as far
46:11that's right you get the idea so kinetic
46:13energy is velocity dependent
46:16squared here's a little device that's
46:18kind of nice it's a portion of a racket
46:21ball and I can invert it when I do that
46:23am I giving it a little energy I'm going
46:25to drop this and watch
46:29did you see that it bounced higher than
46:32when I dropped it HC and here's another
46:35question too will it always bounce
46:38higher from where I drop it if I got
46:40higher up would it still bounce higher
46:43where is the point where it will not
46:44bounce higher anymore and why think
46:47about
46:49that show you another idea with energy
46:51here's a super bow the Super Bowl I'm
46:54going to let it go some people think a
46:56Super Bowl come up to the same height
46:58but it won't it comes up to about a
47:02little bit more than half height okay
47:04this superball is guaranteed to give a
47:06th000 bounces we got this this morning
47:09Troy's been playing with it Troy how
47:10many bounces did you get on it uh 995
47:13995 okay there's 96 97 98 okay let's see
47:19999 1,000 sure enough that's true let's
47:21try it
47:23again a th
47:27its elasticity is gone
47:32gang think about that catch you later
47:40physics trixies
47:48[Music]
47:57I
47:59[Music]

About this transcript

This page contains the full transcript of 08 -- Energy -- Sweet Conceptual Physics By Paul Hewitt by LearnHub, generated from the public captions YouTube serves with the video. The transcript has 7,406 words across 1,073 segments, with the original timestamps preserved so you can click any line to jump to that moment in the embedded player.

What you can do with it

Use the transcript to take notes, quote the speaker, build a study guide, generate a summary with ChatGPT or Claude via the YouTube Summary tool, or export it as a timed subtitle file with YouTube to SRT. You can also re-open it in the transcriber to choose a caption track available on the video.

Free YouTube transcript tool

YouTube2Text is a free YouTube transcript generator — no signup, no daily limit. Paste any YouTube link and get the full transcript instantly, with timestamps, click-to-jump, caption language selection, AI prompts for ChatGPT, Claude, and Gemini, and exports to TXT, SRT, VTT, or Markdown.