# quiz 3 - Version 068/ABABA quiz1031 Demkov (59910) This...

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1 This print-out should have 7 questions. Multiple-choice questions may continue on the next column or page – fnd all choices beFore answering. 001 10.0 points A truck oF mass m T = 2000 kg is going north on Guadalupe with a speed oF 4 m / s. The truck is struck by an eastbound car (mass oF 1000 kg) traveling on 24th street. ±rom the skidmarks aFter the collision, it is deduced that the mangled wreckage, which sticks to- gether aFter the collision was skidding at angle 20 with respect to the initial direction oF the car. What was the initial speed oF the car? 1. 14 m/s 2. 3 m/s 3. 5 m/s 4. 22 m/s correct 5. 10 m/s 6. 17 m/s Explanation: Let : m T = 2000 kg , m c = 1000 kg , v T = 4 m / s , and θ = 20 . m c v c = ( m T + m c ) v x m T v T = ( m T + m c ) v y m c v c m T v T = ( m T + m c ) ( m T + m c ) v x v y = 1 tan θ v c = m T v T m c tan θ = (2000 kg) (1000 kg) (4 m / s) (0 . 364) = 21 . 978 m / s . 002 10.0 points A(n) 7 kg object moving at 10 m / s in the pos- itive x direction has a one-dimensional elastic collision with an object initially at rest. AFter the collision the object oF unknown mass has a velocity oF 10 m / s in the positive x direction. What is the unknown mass? 1. 4.8 2. 3.2 3. 2.5 4. 1.4 5. 8.4 6. 9.0 7. 0.533333 8. 2.4 9. 3.0 10. 7.0 Correct answer: 7 kg. Explanation: Given: v 2 i = 0. ±rom conservation oF momentum, we have m 1 v 1 i = m 1 v 1 f + m 2 v 2 f m 1 ( v 1 i - v 1 f ) = m 2 v 2 f . (1) Because oF elastic collision, energy is con- served. ThereFore 1 2 m 1 v 2 1 i = 1 2 m 1 v 2 1 f + 1 2 m 2 v 2 2 f m 1 ( v 2 1 i - v 2 1 f ) = m 2 v 2 2 f m 1 ( v 1 i - v 1 f ) ( v 1 i + v 1 f ) = m 2 v 2 2 f (2) Dividing Eq. (2) by Eq. (1) we have v 1 i + v 1 f = v 2 f v 1 f = v 2 f - v 1 i (3) Substituting Eq. (3) in Eq. (1) we have m 1 v 1 i = m 1 ( v 2 f - v 1 i ) + m 2 v 2 f m 2 v 2 f = m 1 v 1 i - m 2 ( v 2 f - v 1 i ) m 2 = m 1 [2 v 1 i - v 2 f ] v 2 f (4) = (7 kg) [2 (10 m / s) - (10 m / s)] (10 m / s)

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## This note was uploaded on 11/02/2008 for the course PHY 301 taught by Professor Demovk during the Spring '08 term at University of Texas at Dallas, Richardson.

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quiz 3 - Version 068/ABABA quiz1031 Demkov (59910) This...

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