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Phy 121
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.
** CQ_1_14.1_labelMessages **
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: ->->->->->->->->->->->-> :
I believe it has a 1 newton minimum and 3 newton maximum with an average of 2 newtons. I believe this because if the tension is exerted at 8 and it raises steadily than it stands to reason that 7 cm = 0, 8cm = 1, 9 cm = 2, and 10 cm = 3.
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Good, but tensions less than 1 N certainly occur between the 8 cm and 10 cm lengths. You would be more accurate to average 0 N and 3 N.
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• How much work is required to stretch the rubber band from 8 cm to 10 cm?
answer/question/discussion: ->->->->->->->->->->->-> :
2 newton units of force should be required as this would only be a change of 2 cm which averages at 1 newton per cm based on my previous assumption.
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1 N / cm would be the rate at which the force changes with respect to length, given your assumption that there's a 1 N force at the 8 cm length (0 N would be more appropriate, but let's not worry about that: I'll accept the 1 N for now)..
However that is not the work. What is the definition of work, and how would you calculate the work for this distance?
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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: ->->->->->->->->->->->-> :
The tension is in the opposite direction of the motion as it is pulling towards the opposite direction.
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• Does the tension force therefore do positive or negative work?
answer/question/discussion: ->->->->->->->->->->->-> :
It does positive work because it is providing more force than the tension is pulling with.
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Work is the product of force and displacement. If the displacement and the force are in opposite directions, then they have opposite signs and their product 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: ->->->->->->->->->->->-> :
It should do slightly less than 3 Newtons because it will have resistance from the still domino and will lose energy from any distance required to reach it.
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Any force produced by anything other than the tension does not contribute to the work done by the tension.
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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: ->->->->->->->->->->->-> :
1 newton should be the kinetic energy minus the force required to move the domino.
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1 Newton is a unit of force, not energy.
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• At this point how fast will the domino be moving?
answer/question/discussion: ->->->->->->->->->->->-> :
It will be moving fairly quickly but less quickly than if we stretched to the 10 cm point or more quickly than if it were stretched to 7.5 or 7 cm.
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If you calculate the work and energy correctly, this result can be obtained by setting the expression for kinetic energy equal to the energy.*#&!*#&!
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You've made a number of errors on this problem. I've tried to provide guidance with some notes.
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You did some things well, but from the point where you were asked about the work, you weren't using the right definition. Be sure you have covered all the assignments that precede this problem, as those assignments do include the definitions of work and kinetic energy, as well as the work-kinetic energy theorem. All that is needed to answer most of these questions.
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&#Please see my notes and submit a copy of this document with revisions, comments and/or questions, and mark your insertions with &&&& (please mark each insertion at the beginning and at the end).
Be sure to include the entire document, including my notes. &#
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