1. Introduction to Physical Science Monday, Wednesday, Thursday Tom Burbine tomburbine@astro.umass.edu

2. Quiz #1

3. Momentum Momentum is inertia in motion Momentum = mass x velocity Momentum = mv When you don’t care about direction: Momentum = mass x speed

4. Scalars and Vectors

5. Momentum Moving object can have large momentum if it has a large mass, large velocity, or both

6. To change momentum, you need to change the velocity or the mass of the object Momentum

7. To change the velocity of an object, you need to apply a force, which accelerates an object

8. Momentum The longer the force is applied, The greater the velocity changes, The greater the momentum changes

9. Impulse Impulse = Force x time interval Impulse = Ft

10. Questions: Compare the momentum of a 1 kg cart moving at 10 m/s with that of a 2 kg cart moving at 5 m/s. Does the moving cart have impulse? Does a moving cart have momentum? For the same force, which cannon imparts a greater impulse to a cannonball – a long cannon or a short one?

11. Answers: Compare the momentum of a 1 kg cart moving at 10 m/s with that of a 2 kg cart moving at 5 m/s. Same momentum Does the moving cart have impulse? No. Impulse is something the cart experiences Does a moving cart have momentum? Yes For the same force, which cannon imparts a greater impulse to a cannonball – a long cannon or a short one? Long cannon. Force acts over a longer time

12. Impulse and Momentum Impulse = change in momentum Ft = ∆(mv)

13. Increase Momentum Increase the momentum of an object as much as possible, apply the greatest force as long as possible For example, a golfer and a baseball player will swing as hard as possible and follow through Following through extends the time of contact

14. Exercise

15. Decrease Momentum Why is it better to hit a haystack with a runaway car or a concrete wall to stop? You need the same impulse to stop the car

16. Answer: Why is it better to hit a haystack with a runaway car or a concrete wall if you want to stop? You need the same impulse to stop the car By hitting the haystack instead of the wall, you extend the time during which your momentum is brought to zero

17. Decrease Momentum Break bricks by a large impulse over a short time http://www.youtube.com/watch?v=xwAu0KXTuj

18. Law of conservation of momentum In the absence of an external force, the momentum of a system remains unchanged http://www.youtube.com/watch?v=UwWVEpjAI Dg&feature=relatedhttp://www.youtube.com/watch?v=UwWVEpjAI Dg&feature=related

19. Net Momentum Net momentum (before collision) = Net momentum (after collision)

20. Types of collisions Elastic collision – all the momentum is transferred to another object during a collision Inelastic collision – Colliding objects become entangled

21. Recoil Velocity of a gun The recoil velocity caused by a gun exactly balances the forward momentum of the projectile http://www.youtube.com/watch?v=X8imjiuOx1c

22. Exercise

23. Question: If a freight car travelling at 10 m/s hits a car at rest hits another freight car with the same mass and the frieght cars become stuck together, what will be the velocity of the stuck cars?

24. Answer: If a freight car travelling at 10 m/s hits a car at rest hits another freight car with the same mass and the frieght cars become stuck together, what will be the velocity of the stuck cars? (net mv) (before) = (net mv) (after) (m x 10 m/s) = (2m x v) v = 5 m/s

25. http://dev.physicslab.org/Document.aspx?doctype =5&filename=Compilations_NextTime_Moment um.xmlhttp://dev.physicslab.org/Document.aspx?doctype =5&filename=Compilations_NextTime_Moment um.xml

26. Homework

27. http://www.youtube.com/watch?v=DI3rPT_9rW0

28. http://www.youtube.com/watch?v=DYMTAZrKL cMhttp://www.youtube.com/watch?v=DYMTAZrKL cM

29. Any Questions?