course PHY201 012. `query 12 Physics I 03-07-2008
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01:03:40 Query set 3 #'s 13-14 If an object of mass m1 rests on a frictionless tabletop and a mass m2 hangs over a good pulley by a string attached to the first object, then what forces act on the two-mass system and what is the net force on the system? What would be the acceleration of the system? How much would gravitational PE change if the hanging mass descended a distance `dy?
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RESPONSE --> The force acting on the two mass system is the force of M2 being pulled down by gravity. The acceleration of the system would be the force of gravity on m2 divided by the mass of m1. The gravitational PE would change by the amount of joules represented by the following calculation: (9.8 m/s/s * m1 in kg) * `dy in m confidence assessment: 2
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01:05:04 ** The net force on the system is the force of gravity on the suspended weight: Fnet = m2*9.8m/s/s Gravity also acts on m1 which is balanced by force of table on m1, so this force makes no contribution to Fnet. Acceleration=net force/total mass = 9.8 m/s^2 * m2 / (m1+m2). If the mass m2 descends distance `dy then gravitational PE decreases by - m2 g * `dy. COMMON MISCONCEPTIONS AND INSTRUCTOR COMMENTS: The forces acting on the system are the forces which keep the mass on the table, the tension in the string joining the two masses, and the weight of the suspended mass. The net force should be the suspended mass * accel due to gravity + Tension. INSTRUCTOR COMMENT: String tension shouldn't be counted among the forces contributing to the net force on the system. The string tension is internal to the two-mass system. It doesn't act on the system but within the system. Net force is therefore suspended mass * accel due to gravity only 'The forces which keep the mass on the table' is too vague and probably not appropriate in any case. Gravity pulls down, slightly bending the table, which response with an elastic force that exactly balances the gravitational force. **
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RESPONSE --> self critique assessment: 3
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01:05:32 How would friction change your answers to the preceding question?
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RESPONSE --> Friction would reduce the amount of Fnet by some amount. confidence assessment: 2
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01:05:43 **Friction would act to oppose the motion of the mass m1 as it slides across the table, so the net force would be m2 * g - frictional resistance. **
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RESPONSE --> self critique assessment: 3
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01:07:53 Explain how you use a graph of force vs. stretch for a rubber band to determine the elastic potential energy stored at a given stretch.
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RESPONSE --> The graph of force vs. stretch represents work in the same way that the velocity vs. time graph represents `ds. By taking the area under the trapezoid we can determine work. confidence assessment: 2
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