1. Aim: How can we solve free fall problems? Do Now: A textbook and a metal sphere fall from the same height at the same time. Which one hits the ground first?

2. Gravity – the natural force of attraction in the universe Gravity causes objects to accelerate The acceleration due to gravity is a constant near the surface of the Earth for all objects The elephant and penny hit the ground at the same time because the acceleration due to gravity is a constant

3. Demo: 2 pieces of paper

4. What about crumbled and flat paper? Demo Air resistance makes certain objects fall at different rates The elephant and feather hit the ground at the different times because of air resistance

5. What about in a vacuum (no air resistance)? Demo In a vacuum, all objects fall at the same rate If you travel high in the mountains or go to the moon, the acceleration due to gravity has a different constant

6. Apollo 15 Video

8. Water Droplets with a Strobe light  http://www.youtube.com/watch?v=rvY7N GncCgU

11. What’s Really Happening Time (s)Distance (m)Speed (m/s)Δd (m) 00.0 14.99.84.9 219.6 14.7 344.129.424.5 478.439.234.3 5122.649.044.2 6176.458.853.8

12. Free Fall Problems With all free fall problems, we know two things immediately: 1.The object is dropped from rest v i = 0 m/s 2. Acceleration is due to gravity a = g = 9.8 m/s 2 Solve the problem using the appropriate formula

13. Sample Problems 1.An object is dropped from a height of 2 m. a. How long does it take to reach the ground? Given v i = 0 m/s a = g = 9.8 m/s 2 d = 2 m t = ? d = v i t + ½at 2 2 m = (0 m/s)t + ½(9.8 m/s 2 )t 2 2 = 4.9t 2 4.9 t 2 = 0.4 t = 0.6 s

14. b. How fast is it moving when it hits the ground? Given v i = 0 m/s a = g = 9.8 m/s 2 d = 2 m t = 0.6 s v f = ? v f = v i + at v f = 0 m/s + (9.8 m/s 2 )(0.6 s) v f = 5.9 m/s

15. 2. An object falls off a bridge and takes 2.5 s to hit the water below. a. How high is the bridge? Given v i = 0 m/s a = g = 9.8 m/s 2 t = 2.5 s d = ? d = v i t + ½at 2 d = (0 m/s)(2.5 s) + ½(9.8 m/s 2 )(2.5 s) 2 d = 30.6 m

16. b. How fast is it moving when it hits the ground? Given v i = 0 m/s a = g = 9.8 m/s 2 t = 2.5 s d = 30.6 m v f = ? v f = v i + at v f = 0 m/s + (9.8 m/s 2 )(2.5 s) v f = 24.5 m/s

17.  Pages 68-71