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Old Aug 14th 2013, 03:31 PM   #1
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A few Work-Energy Theorem related problems

I understand the basic principle of the work-energy theorem, but not sure how to factor in friction for these problems. Any help would be greatly appreciated.
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Old Aug 15th 2013, 10:18 AM   #2
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The work done by the person pulling on the rope plus the work done by friction opposing the box's movement must equal the gain in mechanical energy of the box. Remember that work = force times displacement in the direction of that force, so in the case of the person pulling on the rope with force F:

W_person = Fd cos(theta)

where d = displacement

For the work done by friction given the friction force F_f:

W_friction = -F_f d

Note the minus sign, which is due to the fact that the displacement of the box is opposite in direction to the force of friction.

Add these together to get

(Fcos(theta) - F_f)d = change in KE = (1/2)mv^2

Does this help?
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Old Aug 15th 2013, 02:43 PM   #3
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Two Examples

Hello Chippysteve,

Here are two examples that might be of interest.


http://www.thermospokenhere.com/wp/0...te_uphill.html

http://www.thermospokenhere.com/wp/0...ized_work.html

Good luck with your studies... TSH
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Old Aug 18th 2013, 01:57 PM   #4
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Thanks fellas. That helped me out. Whether or not I got the right answer on that homework assignment is a different story.
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