Chapter 6 Momentum

1. Chapter 6Momentum Object A hits Object B. What happens? Physics 1 (Garcia) SJSU

2. Forks In 7th Century royal courts of the Middle East began to use forks at the table for dining but was not until the 16th Century that forks were common in Europe. Early table forks were modeled after kitchen forks but in the late 17th Century forks with four curved tines appear. The additional tines made diners less likely to drop food, and the curved tines served as a scoop so people did not have to constantly switch to a spoon while eating. By the early 19th Century, multi-tined forks were also used in England and slowly began to spread to America. Physics 1 (Garcia) SJSU

3. Force, Momentum, Energy With Newton’s Laws, we can understand motion just using forces. Can also eat food just using knives. Easier to understand motion by introducing concepts of momentum and energy. Think of them as the fork and spoon of mechanics. Physics 1 (Garcia) SJSU

4. Momentum Momentum of an object is, (Momentum) = (Mass) X (Velocity) Examples of objects with large momentums are supertanker (large mass) and bullet (large velocity). Physics 1 (Garcia) SJSU

5. Check Yourself A 2 ton car, going 60 m.p.h. hits a 5 ton truck, going 20 m.p.h.. Which vehicle, the car or the truck, has greater momentum? What would the car’s speed have to be for the momentums to match? Aren’t you forgetting something? How does that matter? Physics 1 (Garcia) SJSU

6. Momentum and Force To stop an object with a large momentum requires either: • Large force (stopping the object quickly). • Small force applied for a long time. Notice that changing object’s momentum depends on force and time interval. Physics 1 (Garcia) SJSU

7. Impulse Define impulse acting on an object as, (Impulse) = (Force on object) X (Time interval) Objects have momentum. Impulse acts on an object. Physics 1 (Garcia) SJSU

8. Impulse & Momentum Impulse is related to momentum by, (Change in momentum) = (Impulse) or (Mass) X (Change in velocity) = (Force) X (Time interval) This relation comes from Newton’s 2nd law. Physics 1 (Garcia) SJSU

9. Demo: Egg Throw Throw a raw egg as fast as possible at a plastic sheet that’s held loosely. X X X X X (Hold here) Physics 1 (Garcia) SJSU

10. LONG TIME small force short time LARGE FORCE Check Yourself Throw egg at sheet or wall with same speed. Which case has: Greater change of velocity? Greater change of momentum? Largest impulse on the egg? Largest time of impact? Largest force on the egg? Physics 1 (Garcia) SJSU

11. (FORCE) x (time) Ouch! X X Demo: Vampire Stake Safest when slow moving stake is placed on a soft, fleshy spot on the chest. (force) x (TIME) Not safe if stake strikes hard skull Physics 1 (Garcia) SJSU

12. Check Yourself A 2 ton car, going 60 m.p.h. hits a 5 ton truck, going 20 m.p.h.. The force of impact is greatest on which vehicle, the car or the truck? The impulse is greatest on which vehicle, the car or the truck? Change of momentum greatest? Change of velocity greatest? Driver injury greatest? Physics 1 (Garcia) SJSU

13. Crumple Zones Air Bags Seatbelts Automobile Safety Maximizing the time of impact on the driver minimizes the force of impact. This principle used in design of: Physics 1 (Garcia) SJSU

14. Collisions When two objects collide, impulse is equal and opposite for the two objects. Before collision IMPULSE IMPULSE Impact After collision Each object has equal and opposite change in momentum. Physics 1 (Garcia) SJSU

15. Momentum Object B After Collision Momentum Object B Before Collision Momentum Object A After Collision Momentum Object A Before Collision + + = A B B A Conservation of Momentum Since change of momentum in a collision is equal and opposite, the momentum gained by one object is the amount lost by the other. Actual amount of momentum exchanged depends on the details of the collision, such as whether or not collision is elastic. Physics 1 (Garcia) SJSU

16. Demo: Elastic Collisions Objects of equal mass exchange momentum on elastic collisions. Physics 1 (Garcia) SJSU

17. Demo: Newton’s Balls Steel balls collide elastically, exchanging momentum on collision. Physics 1 (Garcia) SJSU

18. Demo: Don’t Scratch To sink a billiard ball that is very close to the pocket without having the cue ball go in as well (“scratching”) strike the cue ball hard so it makes a crisp, elastic collision. As with Newton’s balls, cue ball will stop after giving all its momentum to the other ball in the collision. Physics 1 (Garcia) SJSU

19. Demo: Blaster Balls When masses unequal, momentum change can be large. Speed of ping-pong ball is 3x larger (Slingshot effect) Ping pong ball Golf ball Physics 1 (Garcia) SJSU

20. Demo: Inelastic Collisions Objects stick together after colliding. A B A B A B Note: Use this concept in lab experiment entitled “Projectiles” Physics 1 (Garcia) SJSU

21. Check Yourself Large (4 kg) fish swims at 3 m/s towards a small (2 kg) fish and swallows it for lunch. Total momentum before lunch? Total momentum after lunch? Velocity of the large fish (with small fish inside)? Physics 1 (Garcia) SJSU

22. Recoil Momentum conservation also explains recoil (MASS) x (velocity) (mass) x (VELOCITY) Recoil effect is like an inelastic collision in reverse. Physics 1 (Garcia) SJSU

23. Complicated Collisions Collisions at an angle (not head-on) are more complicated. Learn by playing pool. 8 8 8 Physics 1 (Garcia) SJSU