# Homework Help: Centripetal/Tangential Acceleration

1. Dec 13, 2013

### PhysicsVA

1. The problem statement, all variables and given/known data

A 1.2 g pebble is stuck in a tread of a .76-m-diameter automobile tire, held in place by static friction that can be at most 3.6 N. The car starts from rest and gradually accelerates on a straight road. How fast is the car moving when the pebble flies out of the tire tread?

2. Relevant equations

net F= (mv^2)/r
t=I*α
v=ω*r

3. The attempt at a solution
I am self-studying physics right now and encountered this puzzling problem. Because it was in the rolling motions, I immediately tried to solve it using torque. Unfortunately this didn't result in anything, due to absence of distance or time. The only rotational force that dealt with velocity was centripetal, so I considered the force as centripetal force. Using the equation F=mv^2/r, I had (3.6N)=(.012kg)(v^2)/.38m

And finally, v=sqrt((.38m*3.6N)/.012kg)=10.677 m/s...

The textbook says this is wrong. Thus, my question is two-tiered: first, what did I do wrong? second, it looks like this is centripetal force, but why? (Isn't centripetal force inward, thus making friction face outward?)
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Dec 13, 2013

### Dick

Your solution looks fine to me. You've calculated the force needed to keep the stone on the tire at velocity v and equated it to the maximum frictional force.

3. Dec 13, 2013

### rcgldr

Don't forget that gravity is an additional force acting upon the pebble.

4. Dec 13, 2013

### haruspex

Check your conversion of the pebble mass.
Centripetal force is not an applied force. It is the radial component that the net force must have in order to keep the object moving at constant radius. The pebble will be dislodged when the static friction is at its maximum value, yet the radial component of the net force is less than the centripetal force required.
As rgcldr posted, the net force comes from adding the static friction and the gravity on the pebble. However, the gravitational force will be so small that it can be neglected here.

5. Dec 13, 2013

### Dick

Whoa. Thank you. I missed the conversion.