Got another question for everyone.(adsbygoogle = window.adsbygoogle || []).push({});

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

**Physics Forums | Science Articles, Homework Help, Discussion**

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Mechanics pendulum question

**Physics Forums | Science Articles, Homework Help, Discussion**