hw8a - HW#8a Page 1 of 7 Note numbers used in solution...

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HW#8a Page 1 of 7 Note: numbers used in solution steps can be different from the question part. You can practice the methods in the solution and verify with the numbers and answers given in the question part. Or you should practice the methods in the solution and verify your calculation with numbers in your webassign. Problem 1. Find the magnitude of the impulse delivered to a soccer ball when a player kicks it with a force of 1355 N. Assume that the player's foot is in contact with the ball for 5.93 10 -3 s. Solution: Impulse = force * time I = F*t = 1355 *5.93 10 -3 = 8.04kg · m/s Insight: during such a short period of time, because the force is huge, the impulse is not too small. This impulse will change the momentum of the ball. p = I =mv f -mv i , if the ball is at rest to start with its momentum after the kick is mv= I = 8.04kg · m/s, if the mass is 0.2 kg, the v is 40.2 m/s, really fast. Problem 2, On a horizontal frictionless table, masses A and B (2 kg, 3 kg) slide to the right and left, respectively. They have speeds of 3 m/s and 1 m/s, respectively. The two masses collide, and bounce off each other. After the collision, they travel in opposite directions at speeds of 1.5 m/s and 2 m/s, respectively. (Note:In this problem, please use + dir = right, - dir = left, to indicate direction.) a.) Calculate A's momentum: before collision: kg·m/s , after collision: kg·m/s Explain why it is NOT conserved. Solution: Speeds were given, assign directions +/- appropriately: v Aix = + 3 m/s v Bix = -1 m/s v Afx = -1.5 m/s v Bfx = +2 m/s p A is not conserved. It goes from +6 kg m/s before the collision to -3 kg m/s after the collision. The reason it is not conserved is that p A is only conserved when the net force on object A equals zero. But A does feel net force during collision (from B). b.) Calculate B's momentum: before collision: kg·m/s after collision: kg·m/s Explain why it is NOT conserved. p B is not conserved. Similar to part A, p B is not conserved since net force on object B is not zero. B feels a force during collision (from A). c.) Calculate the A+B system's momentum: before collision: kg·m/s after collision: kg·m/s Explain why it IS conserved.
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HW#8a Page 2 of 7 p of A+B system is conserved, since F net sys = 0. The total net force on the system is zero. The force between A and B are internal forces which do not change the total moments of the system. Use p conservation for A+B, since no net external forces on the system: ± N and mg cancel ± Frictionless ± Collision force are internal to system Calculate the kinetic energy of the A+B system: before collision: J after collision: J What kind of collision is this? somewhat inelastic KE decreased. Mechanical energy is lost to heat/deformation. This is a somewhat inelastic collision. Mechanical energy is lost to heat/deformation (but don’t stick together, which would be perfectly inelastic). Somewhat inelastic collision does not stick together. Don’t assume elastic and conservation of KE, when object do not stick together. Only if there is no any lose to deformation, heat,
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This note was uploaded on 09/10/2009 for the course PHY 76875 taught by Professor Turner during the Summer '09 term at University of Texas.

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hw8a - HW#8a Page 1 of 7 Note numbers used in solution...

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