course Phy 201 ǫ|xTDž`ş
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07:12:47 ۫的ņʕҒǕ assignment #012 012. `query 12 Physics I 06-02-2007
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22:46:12 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 Fnet would be the gravity, but the M1 gravity would be balanced out by the M2 gravity force. A=Fnet/mass=9.8m/s^2(M2)/(M1+M2) confidence assessment: 2
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22:50:29 ** 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 understand everything except the mass*accel due to gravity + Tension I guess I just hadn't seen the Tension part before, but I will try to remember this for the future. self critique assessment: 2
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22:55:19 How would friction change your answers to the preceding question?
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RESPONSE --> You would take the net force and subtract the friction from it. confidence assessment: 1
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22:56:49 **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 --> I understand; in this case more specifically, the net force is M2*g, so it woudl be M2*g-Friction. self critique assessment: 2
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22:57:47 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 --> Well, I think you would find the area under the trapezoid, that is, multiply force x stretch length to get the potential energy. confidence assessment: 2
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22:59:20 ** 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 --> I don't think I understand the ""narrow"" trapezoid--does this just mean the area from, per say, a small section or interval of rubber band stretch?
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23:05:50 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 --> Well, the slope would be the Force divided by the stretch, but I am not sure what this represents in terms of other equations. I think this is work *on* the rubber band. confidence assessment: 1
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23:07:14 Query Add comments on any surprises or insights you experienced as a result of this assignment.
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RESPONSE --> GOod, I feel fairly confident with this stuff. self critique assessment: 2
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