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PHY 121 Ch 5b Lecture

# PHY 121 Ch 5b Lecture - Lecture 8 Thursday February 7 1 The...

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1 Lecture 8 Thursday, February 7

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2 The direction of the friction force A. is opposite to the object’s velocity. B. is in the same direction as the object’s velocity. C. opposes the motion of the object’s surface relative to the surface of the contact object . D. Is always opposite to the object’s acceleration.
3 The coefficient of static friction for rubber on concrete is A. 1000 B. 0.001 C. 1 D. 10 -6

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4 The ultimate cause of friction A. is the normal force acting on the object. B. is the weight of the object. C. is the electrostatic forces between surface molecules. D. is the repulsive force between surface atoms.
5 For a skydiver falling in air A. The air drag is proportional to the skydiver’s speed. B. The air drag is proportional to the square of the skydiver’s speed. C. The air drag is a constant force. D. The air drag is proportional to the weight.

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6 Example At an airport, luggage is unloaded from a plane into the three cars of a luggage carrier. The acceleration of the carrier is 0.12 m/s 2 and friction is negligible. The coupling bars have negligible mass. By how much would the tension in each of the coupling bars change if 39 kg of luggage is removed from car 2 and placed in car 1?
7 Constant and variable forces • Some forces always have the same magnitude. For example, the weight is always mg . • Other forces can have different values depending on the circumstances. Examples are the normal force and the friction force.

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8 The normal force I !
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