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Phy 121
Your 'cq_1_15.1' 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_15.1_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?
answer/question/discussion: ->->->->->->->->->->->-> :
The minimum tension at 8cm is zero and it increases with length up to 10cm to 3 Newtons.
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• Assuming that the tension in the rubber band is 100% conservative (which is not actually the case) what is its elastic potential energy at the 10 cm length?
answer/question/discussion: ->->->->->->->->->->->-> :
Since we're increasing PE from zero @ 8 cm to 3 N @ 10 cm, at assumed 100% conservancy, we'd have 3 N cm.
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• If all this potential energy is transferred to the kinetic energy of an initially stationary 20 gram domino, what will be the velocity of the domino?
answer/question/discussion: ->->->->->->->->->->->-> :
a = force / mass
a = 3 N cm / .02 kg = .06 cm/s^2
PE = .03 Joules. When PE = 0 J then KE = .03 J, so:
1/2 m v^2 = ,03 J
v^2 = .03 J / .5(.02 kg)
v^2 = 3 m^2 / s^2
+- sort both sides v = 1.7 m/s
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• If instead the rubber band is used to 'shoot' the domino straight upward, then how high will it rise?
answer/question/discussion: ->->->->->->->->->->->-> :
Fnet = .02 kg * 980 cm/s^2
Fnet = -19.6 Newton cm(opposes motion)
-`dPE = Fnet * `ds
-.03 J = -.196 N * `ds I'm going out on a limb here and thinking that potential energy is - because it's created by stretching the rubber band opposite from intended motion.
-.03 J = -.196 N * .2 m
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`dPE_elastic, the change in the PE of the elastic rubber band, is negative and equal to -.03 Joules..
`dPE_gravitational is positive, equal to +.03 J if we assume that the elastic force is conservative.
At release the domino is stationary, and at its maximum height it is again stationary (assuming it was shot straight up). So `dKE between release and max height is zero.
If the elastic force is conservative, then assuming no other significant nonconservative forces, no work is done by nonconservative forces, and the change in PE must therefore be equal and opposite to the change in KE. No change in KE implies no change in PE.
Conclusion: The elastic PE was converted to gravitational PE.
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&#Good responses. See my notes and let me know if you have questions. &#