# Homework Help: I need help 2 understand MECHANICS

1. Nov 12, 2006

### cherche86

Question from this site! --> http://www3.tsl.uu.se/~tengblad/Energisystem/kursinfo-student/Mekanik/Lab5_prehw.pdf [Broken]

you will be using a track for a marble shaped into a loop. These questions help
you calculate the minimum height to release a marble (assuming no energy loss) so that
the marble makes it around a loop without falling off the track.

When the ball is at the top of the loop, after it has rolled
down the track, there are only two forces acting on it:
the track pushing on it, and gravity pulling on it.

1. What, if anything, will happen to these forces if the
ball is just about to fall off the track? In other words, it is going so slowly that it is
just making it around the top of the loop, and any slower it would fall off.

2. Since the ball is going in a circle, the total force on the ball must be equal to mv2/R.
Use this to calculate the velocity, and then calculate the kinetic energy of the ball when it is just about to fall off the top of the loop.

3. When the ball is at the top of the loop it has both kinetic energy and potential energy.
Write down an equation for the total energy of the ball at the top of the loop.

4. In this calculation, we will assume no energy will be lost when the ball rolls down the
track. Write down an equation for the total energy of the ball when it is released at
the beginning of the track and set it equal to the energy of the ball at the top of the loop. Solve for the minimum height at which the ball must be released to just make it around the loop.

5. The height you just found is the value that theory predicts will allow the ball to just go around the track. When we do this in the laboratory, do you think the ball will have to be let go at a higher, lower, or the same height? Explain why.

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