cq_1_141

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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.

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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: ->->->->->->->->->->->-> : ->->->->->->->->->->->-> :

Minimum tension is 0 Newtons; maximum tension is 3 Newtons. This would make the average

tension 1.5 Newtons.

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How much work is required to stretch the rubber band from 8 cm to 10 cm?

answer/question/discussion: ->->->->->->->->->->->-> : ->->->->->->->->->->->-> :

work = F * `ds

3N * 2 cm = 3 N * .02 cm = 0.06 Joules

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The 3 N force is exerted only at the 10 cm length. It is not applied through the entire

2 cm.

You need to use the average force to get this result.

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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 force would be opposite the direction of motion. The motion is going out

and the tension is pulling it back.

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Does the tension force therefore do positive or negative work?

answer/question/discussion: ->->->->->->->->->->->-> : ->->->->->->->->->->->-> :

That means that the tension force would do negative work.

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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: ->->->->->->->->->->->-> : ->->->->->->->->->->->-> :

The tension force would do -.06 Joules of force

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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: ->->->->->->->->->->->-> : ->->->->->->->->->->->-> :

When the rubber band reaches its 8 cm length the potential energy is .06, so kinetic

energy would be -.06.

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At this point how fast will the domino be moving?

answer/question/discussion: ->->->->->->->->->->->-> : ->->->->->->->->->->->-> :

.5mv^2 = 0.06 J

mv^2 = 2 * 0.06 J

v^2 = .12 J / m

v^2 = .12 J / .02 kg

v^2 = 6

v = 2.4 cm/s

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I don't see any modifications here.

In any case the average force is 1.5 N so the work is 3 N cm which is .03 J, and the velocity will be about 1.7 cm/s.

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