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Ch7_Solutions

# Ch7_Solutions - 7-2 In(d the quicker way(and the way we...

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P121 Week 3 Ch. 7 Homework Solutions page 1 of 2 7-1) a) FALSE. The normal force does no work on the block, regardless of the direction of motion. The normal force acts in the vertical direction, while the motion of the block is always in the horizontal direction. b) TRUE. Gravity actually does no work on the block, for the same reason as the normal force. It is perpendicular to the motion. Therefore, it cannot change the kinetic energy. c) FALSE. The work done by the frictional force is always negative. The frictional force always opposes the direction of motion. d) FALSE. The frictional force always opposes the direction of motion, so it is always negative. e) FALSE. If the force is in the same direction as the velocity, the kinetic energy will increase. However, if the force is in the opposite direction of the velocity, the kinetic energy will decrease.

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Unformatted text preview: 7-2) In (d), the quicker way (and the way we really intend the students to go), is to simply use W=-|Fd| and with W from (c) and F from (a) get d. SO IGNORE THE ABOVE CALCULATION BUT GIVE CREDIT IF THEY DO IT THAT WAY 7-3) a) The force of sliding friction always acts in the direction opposite of the velocity relative to the fixed surface (so it will always act to decrease the kinetic energy of a body sliding on a fixed surface). This means the work is always negative. b) If we move against friction in one direction and then move back in the other direction, we lose kinetic energy both ways. If the friction force were conservative, we would expect to lose energy in one direction and gain it back in the other direction. P121 Week 3 Ch. 7 Homework Solutions page 2 of 2 7-4)...
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Ch7_Solutions - 7-2 In(d the quicker way(and the way we...

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