# Homework Help: Conservation of Energy Help!

1. Oct 18, 2007

### megaforcetkd

This isn't really a homework problem im just trying to figure out how to do it..

I've attached a really badly drawn picture of the setup... But I'll type it out here too.

I have to use a steel ball as a pendulum. A razor blade is attached to a pole on the test so when the string swings past the blade, it cuts the string and the ball flies. There is a cup on the floor and somehow I have to get the ball to fly into the cup.

What equations do I use to figure out how to get it to land in there? I really have no idea =/

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2. Oct 18, 2007

### Staff: Mentor

I assume the problem is: At what angle do you have to release the pendulum so that the ball falls into the cup?

As a first cut, ignore any complications due to energy loss when the string is cut or the height of the cup sides.

Do it in two parts. The second part is just a projectile motion problem: by the time the ball hits the ground, it must move horizontally enough to land in the cup. Use this thinking to figure out the speed of the pendulum at the bottom position.

Then use conservation of mechanical energy to figure out the angle you have to lift the pendulum to get such a speed at the bottom.