Physics Help Forum Apllied Mechanics problem!
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 Jan 19th 2010, 08:25 AM #1 Junior Member   Join Date: Jan 2010 Posts: 11 Apllied Mechanics problem! Hello! i have an applied mechanics problem. dont know how to start off.. or do any other bit for that matter. Can anybody here help me? This one is beyond me =) The figure represents the slow flexion and extension movements of the leg and thigh. the length of the leg is L=0,4m, and the weight of the leg is m=4.8Kg, and the thigh m=9,8Kg. We´re also given the distances to the center of mass of the thigh (B) and leg (D) from the extremities. ( AB= CD= 0.43L ). Determine the force F in function to angle alfa, and the strength of the joint connections of the hip(A), the knee(C) and the ankle(E). EDIT: figure is attached Attempt: AB=CD=0,172m force apllied at c? m.g.sin -alfa? F is going to be the opposite force to the angular velocity at C? Attached Thumbnails   Last edited by Corum; Jan 19th 2010 at 09:04 AM.
 Jan 19th 2010, 09:35 AM #2 Junior Member   Join Date: Jan 2010 Posts: 11 Sorry to bump this, but it's a little bit urgent... i would greatly appreciate some assistance
Jan 19th 2010, 08:02 PM   #3
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 Originally Posted by Corum Sorry to bump this, but it's a little bit urgent... i would greatly appreciate some assistance
Try solving it like this; determine the potential energy. U(s), of the legs as a function of the displacement, s, of the feet. Then take the negative of the derivative and that will give you F. I.e. F = -dU/ds.

 Jan 20th 2010, 05:03 AM #4 Member   Join Date: Jan 2010 Posts: 48 hi everybody I am interested in question but not able to find out answer. Could anybody help me in finding out the solution? I am not able to find out the potential energy of leg (center of mass, D) as a function of alpha. Please help me!!
Jan 20th 2010, 06:14 AM   #5
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 Originally Posted by vyomictor hi everybody I am interested in question but not able to find out answer. Could anybody help me in finding out the solution? I am not able to find out the potential energy of leg (center of mass, D) as a function of alpha. Please help me!!
You need to know how the center of mass of each part of the leg. Then the potential energy of the leg equals the height of that point above the zero reference for potential energy.

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