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I've been looking over some exam past papers for mechanics and I'm stuck on a problem.

Question reads:

"Suppose that a planar pendulum has a weightless rod of length l and a pendulum bob of mass m. The only external force acting on the pendulum is gravity of magnitude mg.

There is a number c such that, if the pendulum bob passes through the downward position with a speed of magnitude > c, it will eventually pass through the upward vertical position, and if it passes through the downward vertical position with a speed of magnitude < c, it will never reach the upward vertical position. Determine the number c. Neglect friction forces in this problem."

There is a diagram that goes with this problem but it's gonna be difficult to draw. Basically comprises of a horizontal y axis (pointing right), a vertical x axis (pointing downwards) and a vector drawn with an angle delta between the vector and the x axis.

I'm fairly certain you have to use conservation of energy but don't really know where to begin. I understand the concept of conservation of energy but don't know how to apply it to problems.

Thanks

Dave

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# Homework Help: Mechanics pendulum question

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