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What mass do you use for the force at P in the x-direction?
I have drawn the free body diagram of the pendulum. I summed the forces in the x and y direction and took the moment about the center of mass of the bar(L/2). I modeled the ball on the end as a point force so that I could do this. I'm sure that is not right and that is why I'm not getting the right answer but I don't know how to change that. I know that alpha and omega are both zero, so the acceleration of the center of mass is equal to aP = ai. Does anyone have any idea what I am doing wrong?
--to Scott-- It is my recommendation that you treat the bar and the point mass as a single FBD. Then you sum forces in x- and y-directions, as you have described. Your moment equation should be about the center of mass of the bar + particle (this is NOT at L/2, rather you need to first find the cm for the bar + particle together). Then solve these three equations together.
Alternately, you can draw individual FBD's of the bar and particle. You will have a force of reaction (both x- and y-components) on the bar due to the particle as well as equal but opposite reactions on the particle due to the bar. You will combine these five equations together in solving. This method requires a few more steps than the first method above since here you have five equations.
Let me know if this does not answer your question.
5 comments:
What mass do you use for the force at P in the x-direction?
I have drawn the free body diagram of the pendulum. I summed the forces in the x and y direction and took the moment about the center of mass of the bar(L/2). I modeled the ball on the end as a point force so that I could do this. I'm sure that is not right and that is why I'm not getting the right answer but I don't know how to change that. I know that alpha and omega are both zero, so the acceleration of the center of mass is equal to aP = ai. Does anyone have any idea what I am doing wrong?
--to Scott--
It is my recommendation that you treat the bar and the point mass as a single FBD. Then you sum forces in x- and y-directions, as you have described. Your moment equation should be about the center of mass of the bar + particle (this is NOT at L/2, rather you need to first find the cm for the bar + particle together). Then solve these three equations together.
Alternately, you can draw individual FBD's of the bar and particle. You will have a force of reaction (both x- and y-components) on the bar due to the particle as well as equal but opposite reactions on the particle due to the bar. You will combine these five equations together in solving. This method requires a few more steps than the first method above since here you have five equations.
Let me know if this does not answer your question.
--to lemerson--
Please see Scott's comment below as well as my comment following his.
If this does not help, let us know where you get stuck.
I suspected that is what I was doing wrong.
How do you find the CM of the bar + particle?
Recall from ME270 that the x-position of a system of two bodies (with masses m1 and m2, and centroids x1 and x2) is given by:
xcm = (m1*x1 + m2*x2)/(m1 + m2)
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