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Lesson_3.3_Homework_Solution

Lesson_3.3_Homework_Solution - Lesson 12 Homework Solutions...

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Lesson 12 Homework Solutions 1. After jumping vertically upward to make a slam dunk, a 70-kg basketball player is essentially in free fall as he returns to the floor. If he falls through a distance of 1.5 m, what is his rate of change of momentum as he returns to the floor? The force on a 70-kg player in free fall is his weight. Weight = 70 kg × 9.8 ms -2 = 686 N Since rate of change of momentum is a measure of the force that causes the change in momentum, the rate of change of momentum = 686 kgms -1 . 2. If you are riding in a car traveling at 72 kmh -1 , what is your momentum relative to (i) the ground? (ii) the car? (i) Your speed relative to the ground = 72 kmh -1 = 20 ms -1 If m is your mass, then the momentum relative to the ground is: P = mv = 20v kgms -1 (ii) Your speed relative to the car is zero. Therefore, momentum relative to the car is zero. 3. A 6.0-g bullet traveling at 200 ms -1 strikes a bullet proof vest and comes to a stop in 500 µ s. What is the average force exerted by the bullet on the vest? Initial velocity of the bullet is: v i = 200 ms -1 Initial momentum P i = mv i = 0.006 kg × 200 ms -1 = 1.2 kg ms -1 Final momentum P f = 0 The change in momentum P = P i – P f = 1.2 kg ms -1 6 P 1.2 The rate of change of momentum = 2400 N t 500 10 = = × The average force exerted by the bullet is a measure of its rate of change of momentum.
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