# Homework Help: Rotational Motion maximum speed

1. Oct 31, 2011

### physgrl

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

3. A 1500 kg car is moving on an un-banked curve with a radius of 98 m. The coefficients of static and kinetic friction are 0.73 and 0.50 respectively. The maximum speed the car can move without skidding is:

*a. 27 m/s

b. 22.4m/s

c. 21 m/s

d. 30 m/s

e. none of the above

2. Relevant equations

Fn*μ=mv2/r

3. The attempt at a solution

I used that formula with the coefficient of kinetic friction (because the object is in motion when it has velocity) and I got 21.9...the answer key says its 27.

2. Oct 31, 2011

### LawrenceC

Have you tried using static friction? Until the vehicle begins drifting, you only have static friction. At every instant in time, the rubber that is in contact with the road surface is not moving relative to the road surface. It's called the instantaneous center.

3. Oct 31, 2011

### Andrew Mason

If the car is not sliding sideways, you would use the coefficient of static friction. The static friction force is perpendicular to the direction of motion at all times.

AM

4. Oct 31, 2011

### physgrl

Ohhh got it i was thinking about friction in the other direction. Thanks