# Yikes, I need help! (Centripital Motion/Forces/Friction)

1. Feb 6, 2008

### tectactoe

I can't solve this problem for the life of me, and I don't know how to begin even. If someone could explain the procedure, I'd be more than grateful... it can't be THAT hard, I just have no idea where to start. =[ Thanks.

"What is the smallest radius of an unbanked (flat) track around which a bicyclist can travel if her speed is 24 km/h and the coefficient of static friction between tires and track is 0.30?"

2. Feb 6, 2008

### John Creighto

This should be posted in the homework subforum. As for how to solve it, first write down what you know to show you are making some effort. Do you know the equation for the force required to accelerate an object in a circle at a given radius?

3. Feb 6, 2008

### tectactoe

"What is the smallest radius of an unbanked (flat) track around which a bicyclist can travel if her speed is 24 km/h and the coefficient of static friction between tires and track is 0.30?"

v = 24 km/h
us = 0.30

I know

(us)(Fn) = m(v^2/r)

but I don't know if that's the right equation, nor how to find r with only these values.

x_x

4. Feb 6, 2008

### John Creighto

You´ll also need the equation for the force due to friction. Once you have these two equations the rest is just algebra.

5. Feb 7, 2008

### ZPower

ok buddy, this is more of a conceptual question. There are only 2 forces acting on the object, the normal force = to its weight, and the Friction force.

=> Sum of Forces in x has to = Friction force

=> ma = -m*(v^2)/r = -(mu)*m*g
Note: negative sign since the vector points inward toward the center of the circle.

Solve and u get
r = (v^2)/(mu)*g

Hope u got it, any more question?

just post them here,

ZPower