# hw7e - HW#7e Page 1 of 2 Problem 1, A 1.80 kg block slides...

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HW#7e Page 1 of 2 Problem 1, A 1.80 kg block slides on a rough, horizontal surface. The block hits a spring with a speed of 2.00 m/s and compresses it a distance of 11.0 cm before coming to rest. If the coefficient of kinetic friction between the block and the surface is μ k = 0.560, what is the force constant of the spring? 415 N/m Solution: E f = E i + Wnc Always true. Here Wnc is not zero, because there is kinetic friction. Here U is ½ k x^2 potential energy stored in the spring. mgh didn’t change. Problem 2, By what factor does an object's kinetic energy change if its speed is doubled? 4 By what factor does its momentum change? 2 Kinetic energy = ½ mv 2 Momentum = m*v Double speed, double momentum. But double speed will make speed Square to be 4 times, hence make kinetic energy by 4 times. It’s like that if you double the edge length of a square shape, the total area becomes 4 times. If you double the radius of a pizza, the total area become 4 times. Next time you will know that a 6 inch pizza only has 36% of area comparing to a 10 inch pizza, right? This kind of problems are very important. If you are confused, first think about if the change is true, whether the asked item will be bigger or smaller. Then consider How many times bigger or smaller.

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## This note was uploaded on 08/25/2010 for the course PHYS 111 taught by Professor Jamesm.lockhart during the Spring '09 term at S.F. State.

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hw7e - HW#7e Page 1 of 2 Problem 1, A 1.80 kg block slides...

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