MOM B - Impulse and Change of Momentum Original Assignment Start with #3. Using the F t = m v Equation to Analyze Impulses and Momentum Changes: 3. Two cars of equal mass are traveling down Lake Avenue with equal velocities. They both come to a stop over different lengths of time. The dot diagrams for each car are shown below. Which car (A or B) experiences the greatest acceleration? Explain. Which car (A or B) experiences the greatest change in momentum? Explain. Which car (A or B) experiences the greatest impulse? Explain. Which car (A or B) experiences the greatest force? Explain. 4. When a boxer recognizes that he/she will be hit by an opposing fist, he/she rides the punch. Use physics to explain why. 5. Mountain climbers use nylon safety ropes due to their tendency to stretch considerably under stress. Use physics to explain why. Consider the diagram at the right for the next three questions. The diagram depicts Before and After velocities of an 800-kg car in two different collisions with a wall. In case A, the car rebounds upon collision. In case B, the car hits the wall, crumples up and stops. Assume that the collision time for each collision is the same. 6. In which case does the car experience the greatest momentum change? 7. In which case does the car experience the greatest impulse? 8. The impulse encountered by the 800-kg car in case A has a magnitude of N s. a. 0 b. 800 c. 3200 d. 4000 e. 7200 f. Not enough information to determine.
9. Evaluate the potential hazard to a passenger involved in a head-on collision in which the two cars stick together compared to when they rebound upon impact. Explain. 10. The diagram below depicts the changes in velocity of a ball that undergoes a collision with a wall. Indicate which case (A or B) has the greatest change in velocity, greatest acceleration, greatest momentum change, and greatest impulse. Support each answer. Greatest v? Explanation: Greatest a? Explanation: Greatest p? Explanation: Greatest F t? Explanation: Computation Problems Classwork 1. *A 1200 kg car accelerates from 13 m/s to 17 m/s. Find the change in momentum of the car. 2. *A small object with a momentum of 6 kg m/s to the west approaches head-on a large object at rest. The small object bounces straight back with a momentum of 5 kg m/s. What is the change in the momentum of the small object? What is the impulse exerted on the small ball? What is the impulse exerted on the large object? 3. *A 0.03 kg golf ball is hit off the tee at a speed of 34 m/s. The golf club was in contact with the ball for 0.003 s. What is the average force on ball by the golf club? 4. *A toy rocket achieves a velocity of 55 m/s after 3 s, when fired straight up. If the average force exerted by the engine is 28 N, what is the toy s mass? 5. *A 15000 kg air jet accelerates from rest to 45 m/s before it takes off. What is the change in momentum of the jet? 6. *A 0.025 kg piece of clay is thrown into a wall and has a speed of 9 m/s before it strikes the wall. Find the change in momentum of the clay and the impulse exerted on the clay if it does not bounce from the wall. 7. *A small bouncy ball with a momentum of 8 kg m/s to the left approaches head-on a large door at rest. The ball bounces straight back with a momentum of 6 kg m/s. What is the change in the momentum of the ball? What is the impulse exerted on the ball? What is the impulse exerted on the door? 8. *A 0.07 kg tennis ball, initially at rest, leaves a racket with a speed of 56 m/s. If the ball is in contact with the racket for 0.04 s, what is the average force on the ball by the racket? 9. *A 0.145 kg baseball reaches a speed of 36 m/s from rest when a bat strikes. If the average force of 500 N was applied on the ball by the bat, what is the impact time? Homework 10. *A 0.17 kg hockey puck slows down from 54 m/s to 35 m/s when it slides on horizontal ice surface. Find the change in momentum of the puck? 11. *A 0.01 kg bullet is fired at 250 m/s into a wooden block that is fixed. The bullet emerges from the block with a speed of 120 m/s. What is the change in momentum of the bullet?
12. *A 0.05 kg tennis ball moves at a speed of 10 m/s and is struck by a racket causing it to rebound in the opposite direction at a speed 16 m/s. What is the change in momentum of the ball? What is the impulse exerted on the ball? What is the impulse exerted on the racket? 13. *A 0.25 kg beach ball rolling at a speed of 7 m/s collides with a heavy exercise ball at rest. The beach ball bounces straight back with a speed of 4 m/s. That is the change in momentum of the beach ball? What is the impulse exerted on the beach ball? What is the impulse exerted on the exercise ball? 14. *A 0.16 kg hockey puck is moving on an icy horizontal surface with a speed of 5 m/s. A player strikes the puck by a hockey stick, after the impact the puck moves in opposite direction with a speed of 9 m/s. If the puck was in contact with the stick for 0.005 s, what is the average force on the puck by the stick? 15. *A constant force of 12 N acts for 5 s on a 5 kg object. What is the change in object s velocity? 16. *A small object with a mass of 1 kg moves in a circular path with a constant speed of 5 m/s. What is the change in momentum during ½ of period; one period?
Answers to Conceptual Problems Using the F t = m v Equation to Analyze Impulses and Momentum Changes: 3. Two cars of equal mass are traveling down Lake Avenue with equal velocities. They both come to a stop over different lengths of time. The dot diagrams for each car are shown below. Which car (A or B) experiences the greatest acceleration? A Explain. The velocity is the same for each car; yet car A stops in the lesser amount of time. Which car (A or B) experiences the greatest change in momentum? same for each Explain. The cars have the same mass and the same velocity. Which car (A or B) experiences the greatest impulse? same for each Explain. The cars have the same momentum (above); since impulse = momentum, the cars must also have the same p. Which car (A or B) experiences the greatest force? A Explain. Force causes acceleration. For the same mass, the greater force is associated with the greater acceleration. 4. When a boxer recognizes that he/she will be hit by an opposing fist, he/she rides the punch. Use physics to explain why. By riding the punch, the boxer extends the time of the collision. This causes the momentum of the fist to occur over a longer period of time. By extending this time, the force is reduced in size. Good thinking! 5. Mountain climbers use nylon safety ropes due to their tendency to stretch considerably under stress. Use physics to explain why. The safety ropes are intended to halt the climber's motion if s/he begins to fall. It exerts an impulse on a falling person to cause a momentum change. Being made of nylon, the time over which the impulse is applied is increased due to the ability of the rope to stretch. By increasing the time, the force is reduced to a safer value. It sure beats hitting the ground. Consider the diagram at the right for the next three questions. The diagram depicts Before and After velocities of an 800-kg car in two different collisions with a wall. In case A, the car rebounds upon collision. In case B, the car hits the wall, crumples up and stops. Assume that the collision time for each collision is the same. A 6. In which case does the car experience the greatest momentum change? A 7. In which case does the car experience the greatest impulse? E 8. The impulse encountered by the 800-kg car in case A has a magnitude of N s. a. 0 b. 800 c. 3200 d. 4000 e. 7200 f. Not enough information to determine.
9. Evaluate the potential hazard to a passenger involved in a head-on collision in which the two cars stick together compared to when they rebound upon impact. Explain. Rebounding is considerably more dangerous because of the greater velocity and momentum. This leads to a greater force and is more hazardous to the passengers. This explains why cars are designed to crumple up during a front-end collision. The crumpling reduces the rebound effect. 10. The diagram below depicts the changes in velocity of a ball that undergoes a collision with a wall. Indicate which case (A or B) has the greatest change in velocity, greatest acceleration, greatest momentum change, and greatest impulse. Support each answer. Greatest v? B Explanation: In Case B, the v is 58 m/s, left.; in Case A, it is only 15 m/s. Greatest a? B Explanation: Assuming the same t, a is greatest when velocity is greatest. Greatest p? B Explanation: For the same mass, the p is greatest for the greatest velocity. Greatest F t? B Explanation: Since impulse= p, it will be greatest for the greatest. p.
Answers Computation Problems 1) p=4800 kgm/s 2) psmall = 11 kgm/s Ismall = 11 kgm/s Ilarge = -11 kgm/s 3) 340 N 4) 1.53 kg 5) 675,000 kgm/s 6) p = -0.225 kgm/s I = -0.225 kgm/s 7) p = 14 kgm/s IB = 14 kgm/s Idoor = -14 kgm/s 8) 98 N 9) 0.0104 s 10) -3.23 kgm/s 11) -1.3 kgm/s 12) p = -1.3 kgm/s Iball = -1.3 kgm/s Iracket = 1.3 kgm/s 13) p = -2.75 kgm/s IBB = -2.75kgm/s IEB = 2.75kgm/s 14) -448 N 15) 12 m/s 16) p1/2 period = -10 kgm/s p1 period = 0