1. Section 2.3 Gravity and Free Fall

2. Acceleration due to Gravity ► ► An object is in free fall if it is accelerating due to the force of gravity and no other forces are acting on it. ► ► Objects in free fall on Earth accelerate downward at 9.8 m/s 2, the acceleration due to gravity.

3. Speed vs. Velocity ► Speed only has a magnitude (60m/s) ► Velocity has magnitude and direction (60m/s downward)

4. We can use this to find speed ► Remember, the acceleration due to gravity (g) is 9.8 m/s 2

5. Practice A penny is dropped from a building. If it takes 15 s for the penny to hit the ground, what is the penny’s velocity when it hits?

6. http://www.youtube.com/w atch?v=TKdtIKRFwoc

7. Practice A penny is dropped from a building. If it takes 15 s for the penny to hit the ground, what is the penny’s velocity when it hits? v = gt = 9.8m/s 2 x 15s = 147 m/s

8. Finding the average speed

9. Practice: same problem as before A penny is dropped from a building. It takes 15 s for the penny to hit the ground. We found that the final velocity was 147 m/s. What is the average velocity?

10. Practice: same problem as before A penny is dropped from a building. It takes 15 s for the penny to hit the ground. We found that the final velocity was 147 m/s. What is the average velocity? v avg = v f + v i = 147m/s – 0m/s = 147m/s = 73.5m/s 2 2 2 2 2 2

11. Find the distance traveled

12. Practice: same problem as before A penny is dropped from a building. It takes 15 s for the penny to hit the ground. We found that the average velocity was 73.5 m/s. What is distance traveled by the penny?

13. Practice: same problem as before A penny is dropped from a building. It takes 15 s for the penny to hit the ground. We found that the average velocity was 73.5 m/s. What is distance traveled by the penny? D = v avg t = 73.5m/s x 15s = 1102.5 m

14. Weight and the 2 nd Law Weight is the force of gravity on an object  Force is the force due to gravity (F g )  Acceleration is the acceleration due to gravity (g)

15. Practice Calculate the weight of a 60-kilogram person (in newtons) on Earth and on Mars (g = 3.7 m/sec2).

16. Practice Answer: 588 N, 222 N

17. So if acceleration due to gravity is the same for everything, why do a feather and a hammer fall at different speeds?

18. Air Resistance Air applies a force upward that resists the force of gravity Shape and size of the object determines the magnitude of the force

19. Terminal Velocity When the force of air resistance equals the force of gravity, the object stops accelerating. This is called terminal velocity

20. http://www.youtube.com/w atch?v=5C5_dOEyAfk

21. 2.3 Review 1. Describe the motion of a freely falling object. Use the words speed, acceleration, and distance in your answer. 2. What is the difference between mass and weight? 3. If you drop a feather and a baseball in a place where there is no air (a vacuum), how will their motions compare? Why?