Mechanical Energy of a Physical Pendulum

[eurekameh]

http://img196.imageshack.us/i/unledup.png/
So we have a physical pendulum. It has a mass of m=200 g and radius 10 cm. It's suspended from point O at a distance h=8 cm. from center C. It is displaced 0.1 rad and released from rest at t=0.

I'm struggling to find the mechanical energy of this pendulum. When it is displaced 0.1 rad before it is released, its mechanical energy is all in the form of potential energy U=mgy.

I've been working on this for a while now; I feel like this should be an easy problem and I'm overthinking it, but I'm making really complicated triangles and doing things like cos(0.1)=x/(R+h) and then (R+h) - x to find y. I'm not getting the correct answer though, which is 7.83 x 10^-4 J.

 

[Sethric]

Have you considered using polar coordinates?

 

[eurekameh]

Nope, didn't think we'd need to. This is for a general physics 1 class.

 

[Sethric]

Ah, I see, I misread. I thought you were looking for equations of motion. Sorry about that. So, what is suspended from O, the mass? What answer are you getting?

 

[eurekameh]

O is the point of rotation. I am getting 1.76 x 10^-3 J.

 

[epenguin]

When it is displaced 0.1 rad before it is released, its mechanical energy is all in the form of potential energy U=mgy.

I've been working on this for a while now; I feel like this should be an easy problem and I'm overthinking it, but I'm making really complicated triangles and doing things like cos(0.1)=x/(R+h) and then (R+h) - x to find y. I'm not getting the correct answer though, which is 7.83 x 10^-4 J.

Hope it works out when you come back after mind-clearing. Think your triangles should be leading you to sines rather than cosines. If all else fails, and as a check anyway, this is a smallish angle, and the sine of a small angle is approximately equal to the angle (in radians).