Can the direction of friction force be opposite to the direction of motion?

[jrk613]

I was wondering how to modify F=ma and put in either static or kinetic friction coefficient. I assume it's something like F-(friction coef) but I'm just not sure, any ideas or websites someone can direct me to? thanks

 

[berkeman]

I was wondering how to modify F=ma and put in either static or kinetic friction coefficient. I assume it's something like F-(friction coef) but I'm just not sure, any ideas or websites someone can direct me to? thanks

When you draw the free body diagram, you put in a friction force vector as one of the force vectors acting on the object. The direction of the friction force vector is to oppose motion of the object (whether static or kinetic friction), and the magnitude is generally F = mu * N, where mu is the coefficient of friction, and N is the normal force between the object and whatever it is rubbing on. N is often just the weight of the object, for example.

 

[sganesh88]

The direction of the friction force vector is to oppose motion of the object (whether static or kinetic friction)

Not the object as a whole. Only the motion of the contact point. For the FBD of a rolling wheel or an entire car for example, friction can turn out to be in the direction of motion.