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Newtons Law 1 & 2 (exam 2)

Newtons Law 1 & 2 (exam 2) - Newtons Law 1 2...

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Newtons Law 1 & 2 Concept Questions http://www.colorado.edu/physics/phys2010/phys2010_sp06/CTWebSolns/CTSolnsIn dex.html CTN-1. A glider is gliding along an air track at constant speed. There is no friction (assume that the air resistance is small enough to ignore). What can you say about the net force (total force) on the glider? A) The net force is zero. B) The net force is non-zero and is in the direction of motion. C) The net force is non-zero and is in the direction opposite the motion. D) The net force is non-zero and is perpendicular to the motion. Answer: The net force on the glider must be zero. Since its velocity is constant, its acceleration is zero. Since its acceleration is zero, the net force on the glider must be zero, according to NII: CTN-2. An astronaut in intergalactic space is twirling a rock on a string. Suddenly the string breaks when the rock is at the point shown) Which path (A, B, C, or D) does the rock follow after the string breaks?
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Answer: C: Before the string breaks, the rock is always moving tangential to the circle. After the string breaks, there are NO forces acting on the rock, so according to Newton's first law, it must move with constant velocity. CTN-3. An object is being lowered on a cord at a constant speed. How does the tension T in the cord compare to the weight mg of the object? A) T = mg B) T > mg C) T < mg
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Answer: T=mg Since velocity = constant, acceleration a = 0. By Newton's 2 nd law, since a = 0, it must be that F net = 0. Since F net = 0, the upward force must exactly cancel the downward force. CTN-4. An object is being lowered on a cord at a speed which is decreasing . There are two forces on the object, the weight, magnitude mg, and the tension, magnitude T, in the cord. What is the direction of the acceleration? A) up B) down C) a=0 Which equation is true: A) T = mg B) T > mg C) T < mg Answers: The acceleration is upward (even though the velocity is downward). T > mg . Since the acceleration is upward, the net force must be upward . This is because of Newton's 2 nd Law: . If the net force is upward, then the upward force T must be greater magnitude than the downward force mg.
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CTN-5. A sailboat is being blown across the sea at a constant velocity. What is the direction of the net force on the boat? A) Left B) Right C) Net force is zero D) Down E) Up Answer: net force is zero. The boat is moving with constant velocity, therefore its acceleration is zero. Therefore, the net force on it must be zero, according to NII: CTN-6. A glider is on a tilted air track and is sliding downhill. What is the direction of the net force on the glider?
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D) None of these. Some other direction. Answer: The net force is always in the same direction as the acceleration. The acceleration is along the track (since the velocity is increasing in the direction along the track), so the net force must be along the track.
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