# Homework Help: Circular Motion and block track

1. Nov 18, 2009

### interxavier

1. The problem statement, all variables and given/known data
A frictionless track contains a circular section of radius R as shown. What is the minimum height at which a block must be started in order for it to go around the loop without falling off the track?

2. Relevant equations
V = r*w
Fr = m*v^2/r = m*r*ω^2

3. The attempt at a solution

I'm sorry I don't have the diagram but you can draw the equation yourselves. I'm considering using the conservation of energy in this problem, but I don't know if it's proper way.

So initially we have:
Ei = 1/2*m*v^2 + m*g*h = 0 + mgH
Ef = 1/2*m*v^2 + m*g*h = 0 + 2mgR

2. Nov 18, 2009

### clamtrox

In particular, you know that the block falls off the track if its speed drops to zero (I'd imagine), and that the speed of the block is the same whenever it's on a given height H.

3. Nov 18, 2009

### Staff: Mentor

You'll need conservation of energy, but that's not all.

The key to this problem is that there is a minimum speed at the top of the track below which the block will lose contact. (That speed is not zero.) To find that speed, analyze the forces acting and apply Newton's 2nd law.