course Phy 121 ??????????????assignment #012012. `query 12
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18:48:29 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 is the gravitational force that is acting on the m2 object that is hanging off the table. The acceleration of the system would be the calculated net force divided by the total mass of the system. The PE would change by how much the distance dy is. confidence assessment: 3
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18:50:16 ** 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 --> ok. self critique assessment: 3
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18:51:50 How would friction change your answers to the preceding question?
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RESPONSE --> If friction were involved you would have to multiply the friction by the total force and then subtract that answer considering the friction is moving in the opposite direction of the force. This will obviously make the acceleration less. The PE should remain the same. confidence assessment: 3
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18:52:01 **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 --> ok self critique assessment: 3
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18:53:07 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 area under the curve is going to tell you the work done. confidence assessment: 3
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18:55:20 ** If we ignore thermal effects, which you should note are in fact significant with rubber bands and cannot in practice be ignored if we want very accurate results, PE is the work required to stretch the rubber band. This work is the sum of all F * `ds contributions from small increments `ds from the initial to the final position. These contributions are represented by the areas of narrow trapezoids on a graph of F vs. stretch. As the trapezoids get thinner and thinner, the total area of these trapezoids approaches, the area under the curve between the two stretches. So the PE stored is the area under the graph of force vs. stretch. **
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RESPONSE --> ok self critique assessment: 3
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18:56:28 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 --> The area represents the work done by the rubberbands because that represents the potential energy. I don't know what the slope tells us. confidence assessment: 2
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18:57:36 ** 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 --> I see how depending on the if the rubber band pulls against an object or if the object pulling on the rubberband tells whether the work is done by or on the system. self critique assessment: 2
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18:58:30 Query Add comments on any surprises or insights you experienced as a result of this assignment.
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RESPONSE --> I liked the intro problems sets in this assignment because it took step by step approaches to solve each problem and gave the formulas for each step. self critique assessment: 3
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