# hw6b' - HW#6b Page 1 of 4 kJ Problem 1 Catching a wave a 75...

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HW#6b Page 1 of 4 Problem 1, Catching a wave, a 75 kg surfer starts with a speed of 1.3 m/s, drops through a height of 2.05 m, and ends with a speed of 8.2 m/s. How much non-conservative work was done on the surfer? kJ Solution. We will calculate the total initial and final mechanical energy, K + U, of the surfer. If the final mechanical energy is less, then negative work was done. This is the usual case, when something like friction or other dissipation results in losing energy. However -- waves could in principle accelerate the surfer! We will have to see . . . . Ef=Ei+Wnc. This is always true. non-conservative 2 1 951 J=0.951 kJ WE E = −= Aha! Positive non-conservative work was done on the surfer by the waves! Resistance, friction etc, do negative work to convert KE into heat. But push and Pull from human, animal, engine, wind, water, etc can do positive work to increase KE or do negative work to decrease KE, depending on whether the force is in the same direction to help the motion or to the opposite direction of the motion. They are all non-conservative work, and the total mechanical energy KE+PE will be changed. Ei is not equal to Ef. Problem 2 , At a playground, a 19.0 kg child plays on a slide that drops through a height of 2.10 m. The child starts at rest at the top of the slide. On the way down, the slide does a nonconservative work of -361 J on the child. What is the child's speed at the bottom of the slide? 1.8 m/s Ef=Ei+Wnc. This is always true.

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HW#6b Page 2 of 4 Problem 3, A 40 kg block slides with an initial speed of 15 m/s up a ramp inclined at an angle of 45 o with the horizontal. The coefficient of kinetic friction between the block and the ramp is 0.5 . Use energy conservation to find the distance along the incline that the block slides before coming to rest. 10.8 m (Hint: You will need to relate the distance slid to the height attained.) Solution: E f = E i + Wnc Here Wnc is not zero, because there is kinetic friction.
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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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hw6b' - HW#6b Page 1 of 4 kJ Problem 1 Catching a wave a 75...

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