1. Free Fall Motion

2. Questions Does every object fall the same? Should every object fall the same? What is a fluid? Do we live in a fluid? What is the composition of our atmosphere? Make a table showing the differences between the Earth and Moon.

3. AIR DENSITY

4. Falling Objects Imagine there is no air resistance… An object moving under the influence of the gravitational force only is said to be in free fall. 4.5-4.6 Free Fall

5. The acceleration of an object in free fall is 9.8 m/s 2. 4.5-4.6 Free Fall Neglecting air resistance!

6. Drop a feather and a hammer on earth and the hammer reaches the floor far ahead of the feather. What about on the Moon? 4.8 Air Resistance and Falling Objects http://history.nasa.gov/40thann/videos.htm

7. A feather and a coin accelerate equally when there is no air around them. 4.8 Air Resistance and Falling Objects Vacuum tube

8. How objects fall without air resistance? 4.8 Air Resistance and Falling Objects F gravity or weight is the only force

9. How objects fall without air resistance? 4.8 Air Resistance and Falling Objects Objects accelerate equally. Why? F gravity or weight is the only force

10. How objects fall without air resistance? 4.8 Air Resistance and Falling Objects Objects accelerate equally. It’s just like on the __________________ Moon Why? No atmosphere means no air resistance (no drag force), so surface area and weight make no difference. All objects fall at the same rate. F gravity or weight is the only force

11. During each second of fall the instantaneous speed of the object increases by an additional 9.8 meters per second. This gain in speed per second is the acceleration. 4.5 Free Fall: How Fast

12. v = gt v represents both speed and velocity. g represents acceleration due to gravity (9.8 m/s 2 ) t represents time the object is free-falling 4.5 Free Fall: How Fast

13. 9.8 m/s 19.6 m/s 29.4 m/s 39.2 m/s 49 m/s 9.8 m/s x t

14. Free Fall An object is said to be in free fall if it is only under the influence of gravitational force. Fg = 100 N This object will fall at a rate of acceleration equal to 9.8 m/s 2. No support or resistance force!

15. Free Fall Physicists consider air resistance to be negligible for heavier objects that fall near the surface of the Earth. Fg = 100 N Fg = 71.2 N Fg = 11 N Don’t worry about air when making calculations!

16. Rising Objects Rising objects decelerate at the same rate that falling objects accelerate. 4.5 Free Fall: How Fast During the upward part of this motion, the object slows from its initial upward velocity to zero velocity. The object is accelerating because its velocity is changing. How much does its speed decrease each second?

17. How objects fall without air resistance? 4.8 Air Resistance and Falling Objects F gravity or weight is the only force

18. How objects fall without air resistance? 4.8 Air Resistance and Falling Objects Objects accelerate equally. Why? F gravity or weight is the only force

19. How objects fall without air resistance? 4.8 Air Resistance and Falling Objects Objects accelerate equally. It’s just like on the __________________ Moon Why? No atmosphere means no air resistance (no drag force), so surface area and weight make no difference. All objects fall at the same rate. F gravity or weight is the only force

20. In Galileo’s famous demonstration, a 10-kg cannonball and a 1-kg stone strike the ground at practically the same time. This experiment demolished the Aristotelian idea that an object that weighs ten times as much as another should fall ten times faster than the lighter object. 6.6 Free Fall Explained

21. F stands for the force (or weight in newtons) m stands for the mass of the cannonball a is the rate of acceleration the weight-to-mass ratio is the same for these or any objects, which means that the acceleration rates are the same. 6.6 Free Fall Explained 1 kg rock 10 kg cannonball 10x gravitational force or weight g = weight/mass a = force/mass

22. Since the ratio of weight (F) to mass (m) is the same for the 10-kg cannonball and the 1-kg stone, they both fall at the same rate of acceleration. 6.6 Free Fall Explained Why?

23. Since the ratio of weight (F) to mass (m) is the same for the 10-kg cannonball and the 1-kg stone, they both fall at the same rate of acceleration. 6.6 Free Fall Explained It takes a larger force to keep the larger mass accelerating because a larger mass has more inertia. Weight-mass ratios for each are identical! Why?

24. All freely falling objects fall with the same acceleration because the ratio of weight to mass is the same for all objects. 6.6 Free Fall Explained

25. AIR RESISTANCE A resistance force caused by air molecules opposing the motion of an object as it moves through the air. A form of friction sometimes called drag.

26. The amount of air resistance force an object experiences depends on the object’s speed and exposed surface area. 6.7 Falling and Air Resistance Air resistance does not depend upon the weight of the object. 1. Speed The greater the speed, the greater the air resistance. 2. Surface Area (exposed or frontal) The greater the surface area, the greater the air resistance.

27. Another variable… How would the composition or density of an atmosphere influence air resistance?

28. Composition of Air Helium Earth’s atmosphere Same concentration, different masses

29. AIR DENSITY More drag Less drag

30. What two factors determine the air resistance force on an object? 6.7 Falling and Air Resistance The speed and the exposed surface area

31. The air resistance force an object experiences depends on the object’s _______ and ___________ _______. 6.7 Falling and Air Resistance

32. think! How fast would a free-falling rock be moving 4.5 seconds after it drops from rest? How about 8 seconds after it is dropped? 4.5 Free Fall: How Fast

33. What is the acceleration of an object in free fall? 4.5 Free Fall: How Fast 9.8 m/s 2

34. For each second of free fall, an object falls a greater distance than it did in the previous second. 4.6 Free Fall: How Far

35. These distances form a mathematical pattern: at the end of time t, the object starting from rest falls a distance d. 4.6 Free Fall: How Far

36. 4.9 m 19.6 m 44.1 m 78.4 m 122.5 m

37. think! An apple drops from a tree and hits the ground in one second. What is its speed upon striking the ground? What is its average speed during the one second? How high above ground was the apple when it first dropped? 4.6 Free Fall: How Far

38. think! An apple drops from a tree and hits the ground in one second. What is its speed upon striking the ground? What is its average speed during the one second? How high above ground was the apple when it first dropped? Answer: 4.6 Free Fall: How Far

39. For a falling object, how does the distance per second change? 4.6 Free Fall: How Far For each second of free fall, an object falls a greater distance than it did in the previous second.