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course Phy 201
7/1 10 am
Masses of 5 kg and 6 kg are suspended from opposite sides of a light frictionless pulley and are released. •What will be the net force on the 2-mass system and what will be the magnitude and direction of its acceleration?
answer/question/discussion: F = 5 kg * -9.8 m/s^2 = -49 Newtons
F = 6 kg * 9.8 m/s^2 = 58.8 Newtons
Fnet = 58.8 Newtons - 49 Newtons = 9.8 Newtons
A = fNet / m = 9.8 Newtons / 11 kg = 0.89 m/s^2
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• If you give the system a push so that at the instant of release the 5 kg object is descending at 1.8 meters / second, what will be the speed and direction of motion of the 5 kg mass 1 second later?
answer/question/discussion: v0 = 1.8 m/s
Force = 5 kg * 9.8 m/s^2 = 49 Newtons
A = 9.8 m/s^2
V = a * t = 9.8 m/s^2 * 1 s = 9.8 m/s
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a * `dt is the change in velocity, not the velocity.
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If the masses were equal the system would be in equilibrium and the acceleration would be 0.
If both masses were on the same side then nothing would be there to counter them, and they would accelerate together at 9.8 m/s^2.
In this situation the masses are closer to being in balance than to being concentrated on the same side. So their acceleration is closer to 0 than to 9.8 m/s^2.
What is their acceleration and what therefore is the correct solution to this question?
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• During the first second, are the velocity and acceleration of the system in the same direction or in opposite directions, and does the system slow down or speed up?
answer/question/discussion: They are in the same direction, and the system speeds up, since there is no friction included in the problem.
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If the lesser mass is descending, would the system speed up or slow down?
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