I am a bit hung up on this fact. Say you have a pendulum and you want to derive its equation of motion. In doing this you will have to look at the force from the weight on the end of the string. If you are using cylindrical co-ords this is what you get for Fg (sorry I don't have the diagram)(adsbygoogle = window.adsbygoogle || []).push({});

Fg = mg(cos(θ)e-sin(θ)_{r}e)_{θ}

Whereeand_{r}eare unit vectors r points along the direction of the string_{θ}

If you were in x and y the weight has only a component in y so I am confused about when you transfer to cylindrical co-ord you get this. Can someone explain. If you do not understand what I am trying to convey I will draw a diagram when I get home today

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# Pendulum question

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