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Old May 10th 2009, 02:48 AM   #1
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Would this be right to say Fc is out of the circle?

I have a question for my engineering assignment, its pretty simple in fact. I just have one small problem..

Here is what I have. There is a spring attached to a stationary point, and a roller that travels down a parabolic curve. This shouldn't matter too much though, you will understand just from the forces.

I'm not sure if Fc should be +ve (into the circle) or -ve (out of the circle).

Forces upwards (+ve):
- Tension from pulling spring

Forces downards (-ve):
- Weight = mg

I need to find the normal (N) of the circle on the slider.

So would it be true to say Fc pushes downwards (out of the circle) and thus is -ve? I don't see how Fc can push into the circle and be +ve. I think about it this way, say I have a piece of string with a rock attached, and swing it in a circular motion. The rock always pulls the string tight and stays to the edge of the circle, considering there is enough speed. So Fc = T, T is into the centre of the circle and Fc pushes out of the circle.

Any explanations or help would be great, if you need a picture of the assignment let me know, but hopefully I've explained it well enough.
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Old May 10th 2009, 03:19 AM   #2
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well, if youre referring to centripetal force, then it is always radially inward

now as for it being positive or negative, that depends on your choice of axes and sign convention as regards direction

ie
if we take radially inward direction to be +ve
then Fc=T+N-mg
if we take the radially outward direction to be positive
then
mg-T-N=-Fc

note that both equations are equivalent
u can gain a better picture by drawing a free body if ur confused about sign convention.

Last edited by Akshay; May 10th 2009 at 03:23 AM.
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