# Bob is pushing on a crate on a flat horizontal rough

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1. Bob is pushing on a crate, on a flat horizontal rough floor. The crate is moving at constant velocity. We conclude that (a) There is no net force acting on the crate 4 (b) Bob must be pushing vertically down on the crate (c) The crate experiences only the action of one force (d) The crate has large inertia (e) We are not using an inertial frame to describe this situation ~ F net ~ F push ~ F friction m~a m d~v dt 0 . B Pushing down would imply the only force in the direction of motion is friction, causing the box to slow or stop. C There are two forces: applied and frictional. D This behavior is independent of mass/inertia. E We are in the frame of the Earth, taken as inertial. 2

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2. The following is a diagram of a bird’s-eye view of a car taking a curve on the road. The car is shown as a small circle and the direction of motion is counterclockwise (as indicated by the arrow on the arc). If the speed of the car is increasing at the instant shown, which arrow best describes the direction of the net force acting on the car? (The arrow is viewed from above, that is, would just be superimposed to the figure above). Above is the force diagram on the car. The centripetal force is blue . The force of the wheels, causing the increasing speed, is red . Their resultant is purple . 3
3. Bob and Alice ride at opposite ends of a diameter of a merry-go-round that is rotating counter-clockwise as seen from above. Alice and Bob sit so they face each other directly across the center of the merry-go-round. Alice has a bucket of golf balls. She rolls one straight at Bob but as far as she can tell the ball veers right and misses Bob. She tries again and the same thing happens. She analyzes the physics of the situation and concludes

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• Fall '07
• Hicks
• Physics, Force, Frame of reference, Alice, ID NUMBER, scantron card

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