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1st place Mousetrap Car Ideas — Transcript

by Mark Rober · 2,929 words · 381 segments · language en · Watch on YouTube

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0:00- This is a mousetrap car.
0:02(funky music)
0:06They're coming for competitions in high school
0:08physics classes, just like the egg drop challenge
0:10or building toothpick bridges.
0:12The goal is to build a car that travels the furthest
0:14or goes the fastest, but in either case, the only power
0:17provided to move the car is from a single mouse trap.
0:21So today I'm gonna show you how to win first place
0:23by building some cars with the world record holder.
0:26And then we're gonna go to the West Coast championships to
0:28see all these principles in action.
0:30And, don't leave.
0:31I know that 99.7% of you have never nor will ever make one
0:36of these, but I will break down in simple terms how
0:38I know this car will go twice as far as this one
0:40and then I'll prove it
0:41and then we'll discuss why
0:42you see these DVD wheels so often.
0:44But do they work and why do some winning cars have
0:47wheels that look like this?
0:48But before we fly all the way out to Texas to
0:50meet the world record holder, I need to lay the foundation
0:52for the one overarching fundamental physics principle
0:55behind the mousetrap car.
0:56It's called mechanical advantage.
0:58And to do that, I'm gonna need my niece and nephews.
1:00I'm gonna bet you guys I could lift my car off the ground
1:03using just my pinkies.
1:05If I can't do it, can have this crisp Benjamin,
1:08but if I can you, guys have to grab me ice cream.
1:11All right, deal?
1:16- I said nothing else but your pinkies.
1:18- I am just using my pinkies.
1:20- No, just your pinkies.
1:21- That's what I'm doing.
1:25This is really good you guys.
1:27Thank you.
1:28If you're willing to move a greater distance
1:30you're able to reduce the amount
1:31of force by a proportional amount.
1:33I can't lift 500 pounds worth of car one time
1:36but I could lift 10 pounds 50 times.
1:39A mechanical advantage is the ratio
1:40of the output force over the input force.
1:43So in this case it's 50.
1:45That means my hand had to travel 50 times further
1:48than just lifting the car in one shot
1:49but the weight was 50 times less
1:51so it was totally worth it.
1:53This principle of mechanical advantage is everywhere.
1:55Let's take a look at a few examples.
1:57If I have four pulleys, that means I have to pull the rope
1:59down four times further than the dumbbell goes up.
2:02But in exchange, it feels four times lighter.
2:04So this has a mechanical advantage of four.
2:06For the ramp, you look
2:07at the ratio of the length to the height.
2:09Your mechanical advantage therefore is 2.2.
2:11That means I have to travel a little further
2:13but the brick should feel 2.2 times lighter pulling
2:15up the ramp versus just pulling the brick straight up.
2:18And sure enough, if you measure each with a scale
2:20this is exactly what you see.
2:22If you think about it
2:23a screw is just a ramp wrapped around a nail.
2:25So here you look at this as traveled
2:27around the thread and divide by the space in
2:29between the threads to get a mechanical advantage of nine.
2:31And as you know, if you really wanna multiply your force
2:33use a ratchet wrench.
2:34Now the distance your hand travels
2:36for one full rotation is 300 times longer than the distance
2:39the screw moves vertically between one thread.
2:41The total mechanical advantage is 300.
2:43It's like a really long short ramp.
2:45So if this scale reads six pounds
2:47the actual clamping force would be 300 times more
2:49or nearly a ton.
2:51And with wheels and axles, it's the same story.
2:53Since this wheel diameter is twice what this one is
2:56as you could probably guess by now
2:58this weight weighs twice as much.
3:00So now we're balanced with a mechanical advantage of two.
3:03And you'll also notice if I move this
3:04the lesser weight travels twice as far.
3:07And finally we have levers
3:09which is where we started with my niece and nephews.
3:11Here, if you compare the ratio
3:12of the distances from the pivot point
3:14we have a mechanical advantage of four, which
3:16of course means I have to move this end four times further.
3:19But it's super easy
3:20because it's one fourth the weight on this side.
3:22And in all of these examples, which you see everywhere
3:25around us, you trade lower force for more distance travel.
3:28This is how humans built amazing things
3:31before all these fancy machines with engines came around,
3:34human muscles are totally strong enough as long
3:36as you're willing to spend a little more distance
3:39to do the task.
3:40And so this principle mechanical advantage is at play over
3:42and over again with the mousetrap cars only in reverse.
3:45It works both ways.
3:46In other words, I don't want the full force
3:48of the spring acting over this tiny distance to act directly
3:51on the wheels or they would spin out.
3:54That would be a very inefficient transfer
3:56of energy from the spring.
3:57So we use mechanical advantage
3:58and make the main lever arm 15 times longer
4:01than the spring lever arm.
4:02And then the wheel diameter is 24 times bigger
4:05than the wheel axle.
4:05So then if we multiply them
4:07our total mechanical advantage is one over 360.
4:10That means the force is 360 times less right here
4:13on the output at the wheels to the floor
4:15versus right here on the input on the spring.
4:18It also means we'll travel 360 times further
4:20than the distance this spring arm rotates.
4:22Alright, so that's enough of a foundation for now
4:24let's go to Texas and meet up
4:26with my buddy Al to build some race cars.
4:28(upbeat music)
4:32- [Man] U.S.A.
4:33(upbeat music)
4:36- [Mark] Not only is he the mousetrap car
4:38world record holder
4:39but he also kind of started the whole thing
4:40and he was Texas high school physics teacher of the year.
4:43And since my dream job is to one day switch
4:45from working as an engineer in the private sector to
4:47go teach high school physics somewhere,
4:48I made him show me all his cool demos.
4:52- I came up with this idea back in 1991 and since
4:56that time I have literally built thousands and thousands
5:00of mousetrap cars myself.
5:01I've seen every possible engineering design
5:04you could ever come up with.
5:05- And there's lots of different variations
5:06for rules for a mousetrap car race.
5:08Let's talk about
5:09how to build the best long distance car first.
5:10For our testing, we started with three identical cars.
5:13The only difference was the length of the leather arm.
5:15So one was short, one was medium, and one was long.
5:19And I've calculated each of their mechanical advantages
5:21which you can see written here.
5:22And given what we know about mechanical advantage
5:24what do you think is about to happen?
5:26- Ready? - Yep.
5:28As you might've guessed, the short lever arm car
5:31takes a strong early lead.
5:32This makes sense because it has the largest mechanical
5:34advantage, therefore the highest force where the wheels
5:37and ground meet.
5:38The downside is that it's a short-lived burst and the medium
5:41and long lever arm cars pass it once it's quickly used
5:44up all its energy.
5:47In the end,
5:47this is how far they each traveled with the longest lever
5:50arm car going the slowest, but making it all the way
5:53to 30 feet.
5:54This brings up the first principle for the long distance
5:56car, to win you want the smallest possible force
5:59over the longest possible distance.
6:01In other words, the smallest fraction
6:03for mechanical advantage possible.
6:05You want your car to be barely creeping forward to waste
6:08as little energy as possible.
6:09You can think of the total energy of the spring
6:11as this amount of water in this cup, and then
6:14this cup represents the amount of energy that's passed
6:17onto your car to move it forward.
6:18If you just quickly dump in all the energy,
6:21a ton spills and splashes out, this will be due to losses
6:24from extra heat generated or even drag force from the wind
6:27which is proportional to your velocity squared.
6:29But if you do it slowly
6:30and more controlled, much more energy
6:36goes to actually moving your car forward.
6:38The next thing we tested was adding graphite to the axles
6:41on all three of the cars and then we race them.
6:45This made a huge difference
6:46and now they went this far,
6:48again the longest lever arm
6:49car won because it was the slowest.
6:51But this shows the importance of dealing with friction.
6:54It's definitely your biggest enemy
6:55with these cars and the friction comes from two spots.
6:58You have the rolling friction
6:59between the wheels and the ground
7:01and then the biggie is between your axles and the car body.
7:04This is why we put the lubricating graphite powder there.
7:06To take our testing a step further
7:08we took the long lever arm car
7:09and added ball bearings in place at the graphite
7:11and that set a new record for us at 50 feet.
7:14So if you only have one hour to make your car
7:16and you wanna have a good showing
7:17you can use a long lever arm like this in conjunction
7:20with the CD wheels to give you a mechanical advantage
7:22of about one over 360
7:25and then use ball bearings at the axles
7:26or just apply some graphite and you're gonna do pretty well.
7:30Next, we figured if long lever arms make it travel slower
7:33and therefore further, we should do a super long lever arm.
7:40But it only made it to here
7:41which was worse than even the short lever arm car.
7:44The problem was that it didn't coast very well
7:47because we had to make it really big
7:48which means it's more heavy, which means more friction.
7:51You know this already intuitively
7:53because it's harder to push a heavy object
7:55on a table than a light object
7:57because there's more friction resisting you.
7:59So principle three is to make it lightweight.
8:01I love this example though
8:02because it shows you need to balance these principles.
8:04If you take any one
8:05of them too far than another principle will creep
8:08in and start penalizing you.
8:09It's an optimization problem and that's what
8:11makes the mousetrap racers such a great project.
8:13That's also why testing is so important.
8:16So tweaking and testing different things
8:17like Al and I did is critical
8:19for honing in on the sweet spot for your specific design.
8:22Next, we tried this big wheel design
8:24which is a popular approach.
8:26The strategy here is the wheel is 56 times larger
8:29than the wheel axle.
8:30So when you combine it with the lever arm
8:32you get a built-in mechanical advantage of one over 840
8:35and that's the equivalent of a lever arm that's two
8:38and a half feet long, but without needing the big heavy car
8:41that seems like a good deal and as such
8:43it was our best car yet and made it all the way to here.
8:46The downside is
8:47that it takes energy to get a big wheel like that rotating.
8:50It's called rotational inertia.
8:51Here's the demo I built to showcase this principle.
8:53These two wheels are identical
8:55except this one has the steel weights placed
8:56at the outer edge of the wheel versus near the axle.
8:59This means it has a higher rotational inertia.
9:01So when we spin them both identically
9:03the one on the right starts spinning faster
9:05and will reach a higher max speed
9:07but the one on the left will coast for longer.
9:09By having bigger heavy wheels, you're basically using them
9:12as a temporary storage for your energy
9:14and then you give it back during the coasting phase.
9:17The problem with this is anytime you transfer energy
9:19you lose some.
9:20Going back to the cups, a little splashes
9:22out each time you pour it, no matter how slowly you do it.
9:26So instead of pouring your spring energy into a big cup
9:29and then eventually getting it back in the coasting phase
9:31it's better just to have reasonable size wheels
9:33and just have one slow pour directly
9:35into the final cup of making your car move.
9:38Additionally, big wheels like this can be hard to steer.
9:40So principle four is to reduce rotational inertia.
9:44This is also why you see people
9:45do this to their wheels sometimes.
9:47It's an effort to keep the wheels large
9:48in diameter to get that built-in mechanical advantage
9:50but to make them weigh less to reduce the energy given
9:53to rotational inertia.
9:54So the final test we ran was Al's world record car
9:56which traveled an astounding 600 feet.
9:59When he set the record, he did some crazy things
10:01like using jewelers bearings on the axles
10:03but the real secret is this pulley here in the middle.
10:06If we look at the ratios
10:07and calculate the mechanical advantage
10:09from the lever, to the pulley, to the wheels
10:11we're looking at one over 4,608.
10:14It's the equivalent of a 16 foot lever arm or a back wheel
10:17four and a half feet in diameter, but without the downside
10:20of the extra weight or wasted rotational inertia,
10:22this thing barely crawls along.
10:25It's hard to even see the spring lever arm moving
10:27as this back wheel spins.
10:28It's really hard to beat a design like this.
10:31And now we'll quickly go through the speed car principle
10:33since most of the same principles apply.
10:35The biggest difference is this time we want to
10:37access all the energy from the spring
10:38in a short burst right at the beginning
10:40because the finish line is only 15 feet away.
10:43So it doesn't make sense to have a really small
10:45mechanical advantage like the pulley car.
10:47Here we want it much closer to the direct force
10:49of the spring itself,
10:50which would be mechanical advantage one.
10:52Problem is if we did that, the wheels would slip.
10:55So you basically want to incrementally increase
10:57your mechanical advantage by making your rear axle thicker
11:00and thicker with tape until your rear wheels start to slip.
11:03Slipping is bad of course
11:04because that's wasted energy because your wheel is spinning
11:06without actually moving your car forward.
11:08It's helpful to zoom in and use the slow-mo
11:10on your phone to see if your wheels are slipping or not.
11:12This means having good traction
11:13on your rear wheels is important
11:15because it means you can have higher forces
11:17before you start to slip.
11:18These squishy foam wheels work great
11:20and just like with the distance car, reducing friction
11:23by using bearings or graphite will definitely help.
11:25As will making it lightweight
11:26because Newton's second law teaches us
11:28that heavier things are harder to accelerate.
11:30Just like you throw a baseball further
11:32than a heavy bowling ball
11:33and smaller diameter wheels not only help
11:35by keeping your mechanical advantage closer to one
11:38but you don't have time to give energy
11:40to these big wheels and then get it back through coasting.
11:43You want all that spring energy to go directly
11:45into making your car go forward.
11:47Okay, so those are the basic principles.
11:49Before we head
11:50to the West Coast Mousetrap Car Championships
11:53I'll just mention I put a list
11:54of 10 practical quick build tips in the video description.
11:57For example, you should soak your bearings
11:59in WD 40 to remove all the grease.
12:01The grease is useful
12:02if the bearings are actually seeing a lot of load
12:04but since he's cars weigh next to nothing,
12:06it's only gonna slow you down.
12:08I should also mention that my buddy Al has
12:09an amazing website called docfizzix.com
12:12where you can buy all the parts I showed today to experiment
12:15and come up with your own unique design.
12:17I'll also put that link below.
12:18Here we go.
12:19(upbeat music)
12:24- [Man] U.S.A.
12:25(upbeat music)
12:30- For this competition
12:30the objective was to travel forward 15 feet
12:32and then return back and stop as close
12:34to the exact same spot you started
12:36in the least amount of time.
12:38Those moves may seem complicated
12:39but you can switch from forward to reverse
12:41by simply switching the direction you wrap
12:43up your axle halfway through, and you can stop
12:45at a certain point by using a wing nut on a threaded axle.
12:48So given the rules
12:49your design choices should foster both speed and precision.
12:53About half the designs relied on preconceived notions
12:55and use CD wheels, which are a real bad choice here
12:58because you're not looking for distance.
12:59They have more rotational inertia and poor traction.
13:02The winning team car
13:03which I won't show here because the finals are next month
13:05really focused on precision.
13:06They used ball bearings, small foam wheels
13:09and they made their car body out of aluminum.
13:11It weighs more than balls of wood
13:12so it did cost more energy to friction, but there's plenty
13:14of energy in the spring for only traveling 30 feet.
13:17So it was worth the trade
13:18off for the extra rigidity and repeatability.
13:20They also told me they tested
13:21and tweaked their design for six months.
13:26It was awesome to hang out
13:27and see all the various design approaches.
13:29Hopefully you learned enough
13:30by now to give you a solid foundation
13:32for your own unique design so you can build, test, tweak
13:36like crazy and then dominate the competition.
13:42Thanks for watching.
13:45(upbeat music)

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