# Friction again

1. May 10, 2007

### f(x)

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

Friction coefficient between an athlete's shoes and ground is 0.9 . Suppose Superman wears these shoes and runs a 50m race.There is no upper limit on his capacity of running at high speeds . Starting from rest, what will be the minimum time before he finishes the race ?

2. My views

I am confused. I read that its the external force friction which allows acceleration to be possible. But friction also opposes relative motion. So in the above situation, is friction the force acting on accelerating Superman or is it retarding him ?

2. May 10, 2007

### H_man

In this case friction is the force that is allowing him to accelerate. If there was no friction between him and his shoes he would no be able to move.

In some cases friction acts against motion, in others (say the contact between a car tyre and the road) it allows movement.

Note... it is not the friction which is pushing him forward, it is the opposition of the friction that is allowing him to push off.

3. May 10, 2007

### f(x)

if i were to increase friction coeff, then acc would also increase ...but how's this possible ?

4. May 10, 2007

### Ahmed Abdullah

The superman is kicking the ground back and racing forward. Frictional force provides him firm grip on the ground. If friction coeff is sufficiently low superman will slip off and break his nose.