- #1
katyushak
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I have a problem that is driving me nuts. It involves a pendulum of length L pulled back an angle theta and released. After it goes through its lowest point, it encounters a peg alpha degrees out and r meters from the top of the string. The mass swings up about the peg until the string becomes slack with the mass falling inward and hitting the peg. I need to find theta through alpha, r, and L.
I guess the easiest way to go about it is through conservation of energy. I found the total energy of the system - it is all potential when pendulum is pulled back (mgh = mg Lcos (theta)), all kinetic at the lowest point, and mostly kinetic at the point alpha degrees off center where it hits the peg (mgLcos(theta) - mg(L-Lcos(alpha) = mv^2/(L-r)). I know the mass will swing certain distance (and a certain angle) past the horizontal axis before the string becomes "slack" and the mass pulls itself inward. I can't calculate the energy at this (highest) point. I know this is the last point where the mass is going in circular motion (radius L-r) that it began after hitting the peg and I know it is going off on projectile down towards the peg from that moment on.
I guess I need to know how it is that the string becomes "slack" - does the tension suddenly go away? Why would that be? Is the sum of forces acting on the body zero at this point so that the body no longer continues in a circle (acceleration not centripetal)? I can't quite put this condition in place. I think if I did, I'd be on the way to solving this. Or am I totally on the wrong track?
Thanks a lot for any hints.
katyushak
I guess the easiest way to go about it is through conservation of energy. I found the total energy of the system - it is all potential when pendulum is pulled back (mgh = mg Lcos (theta)), all kinetic at the lowest point, and mostly kinetic at the point alpha degrees off center where it hits the peg (mgLcos(theta) - mg(L-Lcos(alpha) = mv^2/(L-r)). I know the mass will swing certain distance (and a certain angle) past the horizontal axis before the string becomes "slack" and the mass pulls itself inward. I can't calculate the energy at this (highest) point. I know this is the last point where the mass is going in circular motion (radius L-r) that it began after hitting the peg and I know it is going off on projectile down towards the peg from that moment on.
I guess I need to know how it is that the string becomes "slack" - does the tension suddenly go away? Why would that be? Is the sum of forces acting on the body zero at this point so that the body no longer continues in a circle (acceleration not centripetal)? I can't quite put this condition in place. I think if I did, I'd be on the way to solving this. Or am I totally on the wrong track?
Thanks a lot for any hints.
katyushak
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