Chapter 7_solutions

# Chapter 7_solutions - Chapter 7 Linear Momentum 4. A 30.00...

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1 Chapter 7 Linear Momentum 4. A 30.00 kg mass falls from a height of 4.000 m. The momentum of the mass just before it hits the ground is: A. 144.2 kg m/s. B. 187.8 kg m/s. C. 320.0 kg m/s. D. 442.4 kg m/s. E. 502.1 kg m/s F. 265.6 kg m/s First find the speed on impact: ½ m v 2 = m g h v = (2 g h ) ½ = 8.854 m/s. Then, Momentum p = m v = 265.6 kg m/s 5. A 3.000 kg ball is pitched with a kinetic energy of 20.00 Joules. Then the momentum of the ball is: A. 7.500 kg m/s. B. 8.350 kg m/s. C. 9.450 kg m/s. D. 10.95 kg m/s. E. 12.50 kg m/s. E kin = ½ m v 2 v = (2 E kin / m ) ½ = 3.651 m/s. Then p = m v = 10.95 kg m/s. 6. If the momentum of a ball is doubled, then the kinetic energy is: A. 0.5 times larger. B. 2 times larger. C. 3 times larger. D. 4 times larger. E. 5 times larger. When the momentum is doubled, the velocity is also doubles (mass stays the same). The kinetic energy depends on the square of the velocity, therefore the kinetic energy will be four times larger. 8. A 4.00 kg ball is traveling at 5.00 m/s and strikes a wall. The 4.00 kg ball bounces off the wall with a velocity of 4.0 m/s. The magnitude of the change in momentum of the ball is: A. 8.0 kg m/s. B. 13 kg m/s. C. 26 kg m/s. D. 30 kg m/s. E. 36 kg m/s. p = p f p i = m v f m v i = m ( v f - v i ) = (4 kg) (4 m/s (-5 ms/s)) = (4 kg)(9m/s) = 36 kg m/s. NOTE: the velocities have opposite sign (opposite directions). In this case we took moving away from the wall as the positive direction.

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2 10. A 4.00 kg ball is traveling at 3.00 m/s and strikes a wall. The 4.00 kg ball bounces off the wall with a velocity of 4.0 m/s. If the contact with the wall by the ball lasts for 0.10 seconds, then the average force on the ball is: A. 230 N. B. 280 N. C. 320 N. D. 360 N. E. 400 N. Change in momentum: p = m ( v f v i ) = (4 kg) (4m/s-(-3 m/s)) = 28.0 kg m/s. Average force F = p / t = 280 N. 11. A 5.00 kg ball is moving at 4.0 m/s to the right and a 6.00 kg ball is moving at 3.00 m/s to the left. The total momentum of the system is: A. 2.0 kg m/s to the right. B. 2.0 kg m/s to the left. C. 38 kg m/s to the right. D. 18 kg m/s to the left. E. 20 kg m/s to the right. p = p 1 + p 2 = m 1 v 1 + m 2 v 2 = (5 kg)(4 m/s) + (6 kg) (-3 m/s) = 2 kg m/s (positive, i.e. to the right). 17. An astronaut in a space suit is motionless in outer space. The propulsion unit strapped to her back ejects some gas with a velocity of 50 m/s. The astronaut recoils with a velocity of 1.0 m/s. If the mass of the astronaut and space suit after the gas is ejected is 120 kg, the mass of the gas ejected is:
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## This note was uploaded on 12/23/2011 for the course PHYSICS 2130 taught by Professor Zhixianzhou, during the Fall '09 term at Wayne State University.

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Chapter 7_solutions - Chapter 7 Linear Momentum 4. A 30.00...

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