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# Bullets

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1. Bullets Two bullets of equal mass are shot at equal speeds at blocks of wood on a smooth ice rink. One bullet, made of rubber bounces off of the wood. The other bullet, made of aluminum, burrows into the wood. In which case does the block of wood move faster? Explain.

2. MOMENTUM

3. What is a collision? A collision is a short duration interaction between two objects. An example of a collision includes a train car rolling toward an identical car at rest. The two cars couple together upon impact and then roll down the tracks together.

4. ANOTHER COLLISION The collision between a tennis ball and racket may seem instantaneous to your eye, but that is a limitation of your perception. A careful look at this picture reveals that the right side of the ball is flattened and pressed up against the strings of the racket.

5. The magnitude of the momentum is the product of the object’s mass and speed p=mv The momentum has the same sign as the object’s velocity.

6. Ways to Increase Momentum • Increase velocity: • A fast moving object will have more momentum than a slow moving/still object. • Compare the momentum of a 40kg skateboarder moving 10m/s and a 150kg parked car. • Increase mass: • A larger massed object will have more momentum than a small massed object. • Compare the momentum of a 5kg dog and a 15kg dog both moving at 5 m/s.

7. Summary • A heavy, fast moving object will have a lot of momentum • A light, slow moving object will have very little momentum.

8. Practice Problem #1 • Describe a situation when two objects with very different masses can have similar momenta.(plural for momentum) • Yes…they must be moving….

9. Impulse • A change in momentum is called an impulse or force applied over a time I = ΔFt • What is impulse’s derived unit?

10. Practice Problem #2 • A professional boxer can punch with a force of 150N. When he uses gloves, the time of impact is .9s. However, when he does not use gloves the time of impact is .7s. Calculate the impulse for both punches.

11. Impulse is also known as a change in momentum.

12. THE IMPULSE-MOMENTUM THEOREM The stick exerts an impulse on the puck, changing its speed.

13. The Law of Conservation of Momentum State: Total momentum of a system remains the same before an interaction (a collision or explosion) and after an interaction (a collision or explosion) Mathematically: Momentum before = momentum after pbefore = pafter (mv)before = (mv)after

14. Collisions • Two types of collisions: • Elastic • Ex: • Inelastic • Ex: • Momentum is still conserved in collisions.

16. Practice Problem #3 • A billiards ball, ball A, rolls towards a stationary billiards ball, ball B, with a velocity of 5 m/s. Both balls have a mass of 2kg. • Classify this collision. • Calculate the momentum before the collision. • Calculate the velocity of each ball after the collision. • What direction is each ball traveling in?

17. Practice Problem #4 • A 50kg lineman running at 10m/s tackles a stationary 40kg quarterback. • Classify this collision. • Calculate the momentum before the collision. • Calculate the velocity of the players after the collision. • In what direction are the player traveling in after the collision?

18. Explosions • Opposite of collisions • The particles of the system move apart after a brief, intense interaction • Exploding forces are usually internal

19. 7.1 Momentum • Newton’s 1st Law is also called the Law of Inertia. • Inertia can be applied to objects in motion and at rest. • This chapter inertia is concerned with objects in motion, also called momentum. • Momentum = mass x velocity • An object can have a large momentum if it has a large mass, velocity, or both. • Which as more momentum a car or truck traveling at the same velocity? Why? Truck b/c it has a larger mass • Which has more momentum a car traveling at a faster velocity than a truck? Car b/c it has more velocity • How much momentum does an object at rest have? zero

20. 7.2 Momentum • What has to change if the momentum of an object changes? Mass or velocity • A change is velocity means there is an acceleration. • What is the cause for acceleration? Force • The greater the force applied, the great the acceleration, which means a greater change in velocity, which mean a greater change in momentum. • A force sustained over a great period of time produces more change in momentum. • Both force and time are important in changing momentum. • Impulse = Force x time • The greater the impulse the greater the change in momentum. • You could write another equation: • Force x time = Δ (mass x velocity)

21. 18. Case #1 • Example given: golfer or baseball player • Major Concepts: swinging through allows for more time and an applied force- change in force from instant to instant • Impact= Force (N) • Impulse= impact force x time (Ns) • Case #2 • Example given: hitting haystack instead of wall in a car • Major Concepts: extend impact time and reduce force • When you increase the time of impact, you decrease the force of impact.

22. 7.3 Bouncing • Impulse are great when an object bounces. • The impulse required to bring an object to a stop and then to “throw it back again” is greater than the impulse required to bring the object to a stop. • Explain the physics behind Lester A. Pelton’s improved paddle wheel. • U- turn of paddle wheel • Water “bounce,” so impulse increased

23. 7.4 Conservation of Momentum • Based on Newton’s 2nd Law: a net force produces an acceleration. • The chapter says the same thing only with different key terms. A change in momentum produces a impulse. • Both impulse and force must be exerted on the object by something outside the object, internal forces will not work. • Internal forces come in action/reaction pairs that are equal and opposite to each other in an object. • Newton’s 3rd Law: is about action and reaction forces.

24. Use Newton’s 3rd Law to analyze a cannonball being fired from a cannon. • A. The cannon and cannonball are considered to be internal forces. • B. The momentum before and the momentum after are equal and opposite therefore they cancel each other. • C. The system does not gain ___________ ___________, therefore it does not gain momentum. • If no net force acts on a system, the momentum of that system cannot change. • The Law of Conservation of Momentum states: In the absence of an external force, the momentum of a system remains unchanged.

25. 7.5 Momentum • Net momentum before = net momentum after. • Elastic Collisions = collision without permanently deforming or generating heat • Example: billiards • Inelastic Collisions = collision with deformation and generating heat Ex: a toy car hitting play dough

26. 7.6 Momentum Vectors • Momentum is a vector quality. • Example:

27. Review Questions • Skater has a faster velocity. • Same impulse because same momentum. Impact would be different. • Force and time are inversely related to time. • Yes, no acceleration means no net force which means no momentum. • Yes, because impulse is Ft, or change in momentum

28. 6. Travel away from each other with the same velocity 7. Stick together and zero acceleration 8. The velocity is in the direction of motion, a decreasing to zero