Momentum and Impulse

1. Momentum and Impulse

2. Momentum

3. Momentum • Inertia/mass in motion • Symbol- p • Equation p = mv • Units: kg • m/s • VECTOR inertia In motion

4. A large truck has more momentum than a car moving at the same speed because it has a greater mass. • Which is more difficult to slow down? The car or the large truck?

5. A Thought Experiment: • Suppose that you were captured by an evil physicist who gave you the following choice: • You must either: • Stand in front of a 1000 kg. truck moving at 1 m/s, or • Stand in front of a 1 kg. frozen meatball moving at 1000 m/s. • …think…

6. Plug and Chug p = mv = (2500 kg)(18 m/s) = 45000 kg m/s

7. Plug and Chug 4.68 kg m/s p = mv v = p/m (4.68)/(7.3) = 0.64 m/s

8. Impulse equation • Ft = Δ mv • Impulse is a force applied over a certain time period • An impulse is required to change the momentum of an object. A change in momentum creates an impulse Change in momentum impulse

9. Impulse and Momentum • Impulse = Change in Momentum • = Final (mv) - Initial (mv) • F t = mDv Example:  Wall exertsa force of 10,000 N.The contact time is0.01 s.------------------------------Impulse = F t              = 100 N-s

10. Which would do more damage- stopping a truck, moving at 60 mi/h, by running into: • A brick wall? a hay stack?

11. Why is follow through so important? • A golfer follows through on a swing to increase the ball’s velocity and make it travel farther. • Following through keeps the club head on the ball for a longer period of time. Since time and velocity are directly proportional, increasing the time of contact increases velocity

12. Why does a batter stop the bat during a bunt? • Answer using words and the impulse equation

13. More examples of extended time to decrease force

14. Decreasing time to increase force By swift execution she makes the time of contact very brief and correspondingly makes the force of impact huge! If her hand is made to bounce upon impact, the force is even greater!

15. Which is more likely to break a window? • A rubber ball • A clay ball • Neither Δ v of rubber ball is greater so impulse must be greater

16. Bouncing • Impulses are greater when bouncing takes place • Tis because the impulse required to bring something to a stop and then, in effect, “throw it back again” is greater than the impulse required merely to bring something to a stop

17. Law of Conservation of Momentum • The momentum lost by one object is gained by another object….the total amount is constant or conserved (cannot be created nor destroyed) • Momentum before = momentum after OR… • Momentumintial = momentumfinal • OR Pi=Pf • system – a collection of objects • Closed system – no objects enter or leave • Isolated system – no net external force acts on it

18. Examples • Conservation of Momentum: If there are no external forces, the total momentum for a system remains unchanged. • Example 1: a person sitting inside a car pushing against the dashboard • Example 2: a bullet fired from a rifle

19. Momentum of system is zero Momentum Before = 0-------------MomentumAfter = 0-------------After firing, the oppositemomentacancel.

20. Elastic collision • Objects bounce off each other without deforming or losing energy • Kinetic Energy is conserved

21. Elastic Collisions m1v1i + m2 v2i = m1 v1f +m2v2f **note that there are TWO final velocities**

22. Inelastic Collision • Objects stick together or are deformed with the loss of energy

23. Inelastic Collisions m1v1i + m2 v2i = (m1+m2)vf

24. Momentum Warm up • What is the momentum of an object that is 30kg and is moving at 5 m/s? 2. A 200kg motorcycle slows from 10m/s to 5 m/s. a. What is the change in momentum? b. How much impulse is required to change the momentum? c. If the impulse was applied for 2 seconds, how much force was applied? 3. A huge rugby player rams into a smaller player and knocks him 5m away. What type of collision is this?