Air pressure. — Transcript

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1. 0:00[Music]
2. 0:00This is an airplane in flight.
3. 0:04So what makes it stay up in the air? To
4. 0:07understand the basic physics of flight,
5. 0:10let's have a look at the properties of
6. 0:11air. In particular, air pressure.
7. 0:16Air is made up of many small molecules
8. 0:18like this. Air molecules are made up of
9. 0:21many tiny atoms.
10. 0:24Air may appear invisible, but air has
11. 0:27mass, therefore it has weight.
12. 0:30In the Earth's atmosphere, there are a
13. 0:33multitude of air molecules.
14. 0:36You can think of air as water as it
15. 0:38behaves like a fluid.
16. 0:41Each time an air molecule comes in
17. 0:43contact with an object, it puts pressure
18. 0:45on it.
19. 0:47Let's have a look at this scenario again
20. 0:49and discuss two types of air pressure.
21. 0:52In the case of an object that is
22. 0:54stationary, air exerts static air
23. 0:56pressure on the object.
24. 0:59For instance, how is it possible that an
25. 1:01inflated balloon does not get squashed
26. 1:04by the static air pressure?
27. 1:07It's because the air inside the balloon
28. 1:09balances the pressure outside.
29. 1:13However, should the static air pressure
30. 1:16outside decrease, the balloon will
31. 1:18expand to fill in the space. Oops.
32. 1:23If you force an object onto air, it's
33. 1:26the object's energy that causes air
34. 1:28pressure.
35. 1:30In this case, the object experiences
36. 1:32dynamic air pressure.
37. 1:35For instance, when you stick your head
38. 1:37out of a moving bus, the wind resistance
39. 1:40you feel on your face is dynamic air
40. 1:42pressure.
41. 1:44So, static air pressure plus dynamic air
42. 1:47pressure make up total air pressure.
43. 1:51What this formula suggests is that for a
44. 1:54constant value of total air pressure, if
45. 1:57dynamic air pressure goes up, static air
46. 2:00pressure must go down
47. 2:03and vice versa.
48. 2:06If you observe air flowing through a
49. 2:08tunnel like this, you will notice that
50. 2:10static air pressure is applied to the
51. 2:12walls of the tunnel as represented here
52. 2:14by the red arrows.
53. 2:17For a straight and even tunnel, the
54. 2:19static air pressure is equal throughout.
55. 2:23But if you reduce the width of the
56. 2:24tunnel and observe the air flow,
57. 2:27something interesting happens.
58. 2:30Inside the thinner section of the
59. 2:31tunnel, the speed of the air increases.
60. 2:36Consequently, the dynamic air pressure
61. 2:38in this section increases.
62. 2:40However, static air pressure decreases,
63. 2:43represented here by the small red
64. 2:46arrows.
65. 2:48So in simple terms, the surface area
66. 2:50where the faster air flow is has less
67. 2:53static air pressure.
68. 2:57Perhaps now you can see what we are
69. 2:59getting at.
70. 3:02The shape of the surface in the tunnel
71. 3:04where the lower static air pressure
72. 3:05looks similar to the top surface of an
73. 3:07airplane wing. In part, this is how lift
74. 3:11is created that allows the aircraft to
75. 3:13fly.
76. 3:15by reduced air pressure above the wing
77. 3:17compared to the higher air pressure
78. 3:19below the wing. As long as the plane is
79. 3:22moving fast enough through the air,
80. 3:25[Music]