course Phy 121 wӣΜֻe٩assignment #012
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12:01:20 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 net force on the system would be calculating the force of gravity exerted on the m2 because it is hanging vertically. Fnet = m2 * 9.8m/s^2 confidence assessment: 1
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12:02:21 ** 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 --> I didn't know the second part, but I know now. self critique assessment: 3
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20:52:12 How would friction change your answers to the preceding question?
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RESPONSE --> To find the answers when friciton is present you multiply the mass on the table by the gravitational force which would be calculated as follows: m1 * 9.8m/s^2 = ? Newtons Now we multiply that answer by the amount of friction which gives us ?Newtons. Now we Subtract the amount of Newtons of m1 by the amount of Newtons due to friction. confidence assessment: 3
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20:56:32 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 --> As the rubber band is stretched gaining PE, we can see that the more it is stretched the more the PE it gains. By graphing this information we can plot PE and determine its amount. confidence assessment: 1
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20:57:34 STUDENT QUESTIONS: Does the slope of the F vs stretch graph represent something? Does the area under the curve represent the work done? If so, is it work done BY or work done ON the rbber bands?
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RESPONSE --> confidence assessment:
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20:58:24 ** Slope isn't directly related to any physical quantity. The area is indeed with work done (work is integral of force with respect to displacement). If the rubber band pulls against an object as is returns to equilibrium then the force it exerts is in the direction of motion and it therefore does positive work on the object as the object does negative work on it. If an object stretches the rubber band then it exerts a force on the rubber band in the direction of the rubber band's displacement, and the object does positive work on the rubber band, while the rubber band does negative work on it. **
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RESPONSE --> OK self critique assessment: 3
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20:58:37 Query Add comments on any surprises or insights you experienced as a result of this assignment.
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RESPONSE --> None self critique assessment: 3
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