Homework Help: Friction Force FBD

1. Apr 6, 2014

tmobilerocks

1. The problem statement, all variables and given/known data
What is the greatest acceleration that can be generated by a runner if the coefficient of static friction between shoes and road is 0.95?

2. Relevant equations
Fnet = ma
Force of static friction: usN
Weight= mg

3. The attempt at a solution
FBD: Positive x axis is to the right. Positive y axis is to the top. Normal force pointing up, equal in magnitude to weight pointing down. Friction must point in the positive x direction.

f = max
ax = usg
amax = (0.95*9.81)
= 9.3 m/s2

I just have one question. Does the friction force point to the right, in the direction of the positive x axis? Thanks!

2. Apr 6, 2014

BvU

To answer your one question yourself you can imagine trying to accelerate on an (almost) friction-free surface, such as ice.

3. Apr 6, 2014

tmobilerocks

So the friction force would point to the right? Due to Fnet = ma, the acceleration and net force should be in the same direction. The only way this is possible is the friction force pointing to the right.

4. Apr 6, 2014

BvU

Yes, if the runner is trying to accelerate in the positive x direction (I didn't find that all too clearly in the story...). Runner pushes backward, exercises a force in the -x direction. No slipping means there must be a force in the +x direction to offset it: the friction force.

Compare with car braking: friction slows it down. Accelerating: friction allows it to accelerate.