Physics Ch.2.1 Part 2 Air resistance and terminal velocity — Transcript

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1. 0:01[Music]
2. 0:18dark blue okay get your paper and your
3. 0:20ball to hit the ground at the same time
4. 0:23a common activity to do when discussing
5. 0:25Gravity the uncoupled paper is held up
6. 0:27by the air around it if we change the
7. 0:30shape to be similar to the ball we can
8. 0:32get them to fall at the same rate the
9. 0:34crumpled paper can move more easily
10. 0:36through the air it now has less surface
11. 0:39area for the air to resist these objects
12. 0:42were difficult to get to fall at the
13. 0:43same time it seems to go against
14. 0:45Galileo's Discovery actually Galileo
15. 0:48realized that air can affect the way an
16. 0:50object falls so he said objects pick up
17. 0:53speed at the same rate regardless of
18. 0:56weight when gravity is the only force
19. 0:58acting on it when objects Falling
20. 1:01Towards the Earth they may have
21. 1:02different speeds this is because the air
22. 1:04resists the fall of objects and those
23. 1:07that are lightest with respect to the
24. 1:09size will be the most effective here is
25. 1:13a book and a piece of paper the book
26. 1:15weighs much more than the paper and when
27. 1:17they're released the book hits the
28. 1:18ground first but if we put the paper on
29. 1:20top of the book so that the air pressure
30. 1:22under the book can't affect and resist
31. 1:25the fall of the paper the two objects
32. 1:27fall
33. 1:28together
34. 1:31objects in a vacuum where there's no air
35. 1:33or other gases present would fall at the
36. 1:36same Speed Most of the universe is made
37. 1:38up of the vacuum of
38. 1:41space well in my left hand I have a a
39. 1:44feather in my right hand a hammer and I
40. 1:47guess one of the reasons uh we got here
41. 1:49today was because of a gentleman named
42. 1:51Galileo a long time ago who made a
43. 1:53rather significant discovery about
44. 1:55falling objects in gravity fields and we
45. 1:58thought that uh where would be a better
46. 2:00place to confirm his uh findings and on
47. 2:04the
48. 2:05moon and uh so we thought we'd try it
49. 2:07here for you the feather happens to be
50. 2:10appropriately a falcon
51. 2:11feather for our Falcon and I'll drop the
52. 2:15two of here and hopefully they'll hit
53. 2:17the ground at the same
54. 2:20time how about
55. 2:23that proves that Mr Galileo was correct
56. 2:26in his
57. 2:28findings
58. 2:32an object is in freef Fall Only If
59. 2:35gravity is pulling it down and no other
60. 2:37forces are acting on it because air
61. 2:39resistance is a force freef fall can
62. 2:42occur only where there is no air
63. 2:45skydivers are often described as being
64. 2:47in freef Fall before they open their
65. 2:49parachutes however that is an incorrect
66. 2:51description because air resistance is
67. 2:53always acting on the sky diver check out
68. 2:56this Discovery Channel video that
69. 2:58discusses a skydivers fall and
70. 3:00introduces the term terminal
71. 3:12velocity when a skydiver jumps out of a
72. 3:15plane at 3,000 M her speed goes through
73. 3:18some big
74. 3:21changes so what's happening to the
75. 3:23forces as you fall through the
76. 3:28air
77. 3:36the force of gravity is always trying to
78. 3:38pull us towards the
79. 3:44Earth before the sky diver jumps the
80. 3:47floor of the plane provides a counter
81. 3:49Force which balances the downward force
82. 3:51of gravity right up until the moment she
83. 3:53steps
84. 3:58out
85. 4:02gravity immediately pulls her towards
86. 4:04the ground and she starts to accelerate
87. 4:07downwards but air resistance is acting
88. 4:10in the opposite
89. 4:13direction the faster she falls the
90. 4:15greater the air
91. 4:18resistance eventually the upward force
92. 4:20of the air equals the downward force of
93. 4:24gravity when forces are balanced the
94. 4:27speed remains constant that doesn't mean
95. 4:30she stopped falling it means she stopped
96. 4:33accelerating she's still doing a steady
97. 4:3655 m/s and hitting the ground at this
98. 4:39speed isn't a good
99. 4:43idea to slow down the upward Force needs
100. 4:46to be greater than the downward
101. 4:50Force when the parachutes open air
102. 4:53resistance suddenly becomes much greater
103. 4:55than
104. 4:57Gravity the forces are no longer back
105. 4:59balanced and this changes her speed she
106. 5:02slows
107. 5:08down because she's slowing down air
108. 5:11resistance decreases until it balances
109. 5:13gravity again and she reaches a much
110. 5:16slower constant
111. 5:22speed approaching the ground at 5 m/s is
112. 5:25a much better
113. 5:28idea
114. 5:34as she hits the ground there's a
115. 5:35thumping great counter force from the
116. 5:37earth which decelerates her very
117. 5:56quickly so for skydivers to land safely
118. 5:59they need to understand forces and use
119. 6:01air resistance to slow themselves
120. 6:07down okay we're going to discuss a
121. 6:09little bit more about the sky diver and
122. 6:11the speed of a falling object and how it
123. 6:13increases and how the air resistance in
124. 6:17turn will increase as well so if we look
125. 6:19at the initial fall of the sky diver we
126. 6:21see that gravity is pulling the sky
127. 6:23diver down and air resistance is going
128. 6:26to be low we see that gravity is greater
129. 6:29than air resistance and the net
130. 6:30direction will be down the object's
131. 6:33going to quickly gain velocity and start
132. 6:36fing faster and faster and as they do
133. 6:39that they're going to encounter more and
134. 6:40more air particles or air resistance and
135. 6:43that's going to affect their fall uh the
136. 6:46upward Force which is the air resistance
137. 6:47is going to continue to increase until
138. 6:50it ends up in midf Fall becoming equal
139. 6:53to the downward force of gravity the net
140. 6:56force is going to end up being zero
141. 6:58because both forces are equal and they
142. 7:01cancel each other out the object's going
143. 7:03to stop accelerating at this point it
144. 7:05doesn't mean that the object's going to
145. 7:07freeze in the middle of the air but it's
146. 7:09going to stop going faster and
147. 7:11faster uh it terminal velocity has been
148. 7:14reached and just to kind of get it in
149. 7:16our notes terminal velocity the
150. 7:18definition is the constant velocity of a
151. 7:20falling object when the force of air
152. 7:23resistance is equal in magnitude in
153. 7:25opposite in direction to the force of
154. 7:28gravity so both of those are the red and
155. 7:30the yellow are going to be equal and
156. 7:32that's how you know that you've reached
157. 7:33terminal velocity a skyd actually looks
158. 7:35at her accelerometer that's usually
159. 7:37around her waist and she could see that
160. 7:40her needle for her velocity is going to
161. 7:42stay at that constant speed so that's
162. 7:44how she knows she reached termal
163. 7:46velocity and we'll go through this a
164. 7:48little bit more during class and then we
165. 7:50know that the object's going to continue
166. 7:52to fall but at a constant velocity and
167. 7:55they're not going to speed up anymore
168. 7:56they're not going to get faster and
169. 7:58faster once she hits termal velocity
170. 8:00that's when she knows that she needs to
171. 8:02pull her parachute so gravity is going
172. 8:05to stay the same in all and throughout
173. 8:07the whole sit um the fall gravity is
174. 8:09going to stay the same so notice my red
175. 8:11arrow is the same size every time but
176. 8:13once they pull that parachute air
177. 8:15resistance is huge um you're catching
178. 8:18all this air trapping all this air so
179. 8:21air resistance is going to be greater
180. 8:22than Gravity the net direction is going
181. 8:26to be up and the objects velocity is
182. 8:30going to slow down drastically and this
183. 8:32is going to help the skydiver um able to
184. 8:36land they're going to land around 5 m/
185. 8:38Second which is much more doable than
186. 8:40probably the 55 m/ second that they were
187. 8:42once going in the middle of their
188. 8:44fall we're going to discuss more about
189. 8:46terminal velocity and we're going to
190. 8:48draw another picture of a skydiver when
191. 8:49you come to class but thank you very
192. 8:51much for watching there's a couple of um
193. 8:54questions that I want you to answer but
194. 8:56I appreciate your
195. 8:58attention
196. 8:59[Music]