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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: ->->->->->->->->->->->-> :
#$&* At 8cm = 0 Newtons 10cm = 3 Newtons tAve = 3 N / 2 = 1.5 N
• How much work is required to stretch the rubber band from 8 cm to 10 cm?
answer/question/discussion: ->->->->->->->->->->->-> :
#$&* For work, I need to know displacement and Fnet.
`ds = 10 cm - 8cm = 2cm Fnet = 1.5 N
`dW = 1.5N * 2cm = 3.0 N cm
• During the stretching process is the tension force in the direction of motion or opposite to the direction of motion?
answer/question/discussion: ->->->->->->->->->->->-> :
#$&* Its in the same direction as the motion.
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The tension force pulls back against the direction of motion. The force you exert is in the direction of motion, opposite to the tension force.
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• Does the tension force therefore do positive or negative work?
answer/question/discussion: ->->->->->->->->->->->-> :
#$&* The tension force does positive work because it's positive, as is the displacement of the stretching motion.
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: ->->->->->->->->->->->-> :
#$&* Again, .03 J. When the rubber band is stretched, - force is applied. When it's let go, + force brings it back to 8cm.
• Assuming this is the only force acting on the domino, what will then be its kinetic energy when the rubber band reaches its 8 cm length?
answer/question/discussion: ->->->->->->->->->->->-> :
#$&* .3 J across the board. The rubber band returns to its original form and exerts .3 J of energy onto the domino.
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It's the tension force that does work on the domino.
If the tension force does positive work after release, then it must have done negative work during the stretching.
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• At this point how fast will the domino be moving?
answer/question/discussion: ->->->->->->->->->->->-> :
#$&* KE = 1/2 m v^2,
*#&!
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You know that the KE is .03 J.
Assuming a 15 gram domino you would expect it to be moving at around 2 m/s.
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*#&!*#&!
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Your work-energy quantities are all of correct magnitude, but there is an inconsistency regarding the signs. If the tension force does positive work in one direction it does negative work in the other.
Check my notes above and thing everything through before looking at the discussion below.
&#See any notes I might have inserted into your document, and before looking at the link below see if you can modify your solutions. If there are no notes, this does not mean that your solution is completely correct.
Then please compare your old and new solutions with the expanded discussion at the link
Solution
Self-critique your solutions, if this is necessary, according to the usual criteria. Insert any revisions, questions, etc. into a copy of this posted document. Mark any insertions with &&&& so they can be easily identified.If your solution is completely consistent with the given solution, you need do nothing further with this problem. &#
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