# Centripetal Force through Turns

1. Sep 3, 2011

### RiskX

Hi,

I face some difficulties trying to solve the following problem:

"You’re sitting in the passenger seat of the car, approaching
a tight turn with a 10.0-meter radius. You know that the coefficient of static
friction is 0.8 on this road (you use the coefficient of static
friction because the tires aren’t slipping on the road’s surface) and that the
car has a mass of about 1,000 kg. What’s the maximum speed the driver can
go and still keep you safe?"

I didn't understand why we use the coefficient of static fricftion if the car is already on a run... Is that beacuse the surface has changed from a stright plane to a curved one?

2. Sep 3, 2011

### stallionx

Because it is that friction force that keeps the car in circular motion.

3. Sep 3, 2011

### gonzacf

I am not sure but I guess you mean why use the static coeficient instead of the dynamic.

Note that the wheels are spining and in fact their surface is statically adhered to the road surface. Only if the car looses grip (for example going too fast in the turn) and starts to slide the coeficient changes.

4. Sep 3, 2011

### stallionx

The dynamic friction as the car rotates along the circle should equal the driving motor force ( in opposite direction ) for tangential acceleration to be zero.

So that uniform circular motion criteria are met.