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Unformatted text preview: homework 15 RAHMAN, TARIQUE Due: Mar 29 2008, 3:00 am 1 Question 1, chap 11, sect 2. part 1 of 1 10 points Railroad car A rolls at a certain speed and makes a perfectly elastic collision with car B of the same mass. After the collision, car A is observed to be at rest. How does the speed of car B compare with the initial speed of car A? 1. The speed of car B is more than the initial speed of car A. 2. The speed of car B is less than the initial speed of car A. 3. The speed of car B is the same as the initial speed of car A. correct 4. Cannot compare since the number of the energy is not conserved. Explanation: Since the collision is a perfectly elastic col- lision, the total momentum of car A and B must be conserved before and after collision. So m A v A = m B v B . Since m A = m B , v B = v A . Question 2, chap 11, sect 2. part 1 of 1 10 points A(n) 23 . 9 g bullet is shot into a(n) 4508 g wooden block standing on a frictionless sur- face. The block, with the bullet in it, acquires a velocity of 1 . 68 m / s. Calculate the velocity of the bullet before striking the block. Correct answer: 318 . 56 m / s (tolerance 1 %). Explanation: Basic concepts: Momentum of any object is p = m v . Solution: The collision is inelastic, and by conservation of momentum p before = p after m b v b + 0 = ( m b + m w ) v f v b = ( m b + m w ) v f m b . Question 3, chap 11, sect 2. part 1 of 1 10 points A car m c = 1520 kg crashes into a wall with the cars velocity immediately before the collision is v = 13 . 6 m / s. The bumper is being compressed like a spring with the spring constant k = 1 . 14 10 7 N / m. The maximum deformation of the bumper for this collision is: Correct answer: 0 . 157039 m (tolerance 1 %). Explanation: Set the potential energy of the bumper after the deformation to be equal to the initial kinetic energy, 1 2 m c v 2 = 1 2 kx 2 , = x = radicalbigg m c v 2 k . Question 4, chap 11, sect 2. part 1 of 2 10 points A(n) 10 g object moving to the right at 35 cm / s overtakes and collides elastically with a 27 g object moving in the same direction at 19 cm / s....
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