Phy 231
Your 'cq_1_14.1' report has been received. Scroll down through the document to see any comments I might have inserted, and my final comment at the end.
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A rubber band begins exerting a tension force when its length is 8 cm. As it is stretched to a length of 10 cm its tension increases with length, more or less steadily, until at the 10 cm length the tension is 3 Newtons.
• Between the 8 cm and 10 cm length, what are the minimum and maximum tensions, and what do you think is the average tension?
answer/question/discussion: ->->->->->->->->->->->-> :
0 N at 8 cm to 3 N at 10 cm
knowing this, the average tension is probably about (0 N + 3 N)/2 = 1.5 N
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• How much work is required to stretch the rubber band from 8 cm to 10 cm?
answer/question/discussion: ->->->->->->->->->->->-> :
10 cm – 8 cm = 2 cm
average tension/force = 1.5 N
work = f * ds
work = 1.5 N * 2 cm
work = 3 N
Good, but 1.5 N * 2 cm = 3 N * cm, not 3 N
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• During the stretching process is the tension force in the direction of motion or opposite to the direction of motion?
answer/question/discussion: ->->->->->->->->->->->-> :
It is in the direction of motion
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• Does the tension force therefore do positive or negative work?
answer/question/discussion: ->->->->->->->->->->->-> :
The tension is the opposite, and the stretching force is positive so then the tension force is negative.
The tension force could be positive or negative depending on the direction you choose as positive.
However the question asked about the work done by the tension force.
Either direction could be chosen as the positive direction; the point is that the tension force and the displacement are in opposite directions, so the work done by the tension force is negative.
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The rubber band is released and as it contracts back to its 8 cm length it exerts its tension force on a domino of mass .02 kg, which is initially at rest.
• Again assuming that the tension force is conservative, how much work does the tension force do on the domino?
answer/question/discussion: ->->->->->->->->->->->-> :
This means the work is the same as the stretching force because it is conservative. Therefore, the result is once again 3 N.
3 N * cm would be correct
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• Assuming this is the only force acting on the domino, what will then be its kinetic energy when the rubber band reaches its 8 cm length?
answer/question/discussion: ->->->->->->->->->->->-> :
Fnet = KE, Fnet = 3 N
KE = 3 N
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• At this point how fast will the domino be moving?
answer/question/discussion: ->->->->->->->->->->->-> :
ke = ½ * m * v^2
0.03 J = ½ * 0.02 kg * v^2
v = 1.73 m/s
.03 J is correct; it's not clear how you connected this with your result 3 N, but it's unlikely to have been an accident, so it appears you know what you're doing here. See the link given below, just to be sure.
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45 minutes
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No revision is requested unless you have questions.
&#Please compare your solutions with the expanded discussion at the link
Solution
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