TITLE: Average friction force for an object going up a slope
QUESTION [0 upvotes]: What is the difference between frictional force and average force of friction?
I know we calculate frictional force in a typical slope problem by finding out the forces that act in opposition, most commonly $F_f = mg\sin\theta$ in most slope problems.
A problem is asking me to find average force of friction, and the method mentioned above is not working, so I assume that average force of friction is a whole different value. My peers have said it is calculated by using $W = F*d$ (given the values of $W$ and $d$) Why are these two different forces a different value? What is different about average force of friction?

REPLY [1 votes]: A value of a force is usually an instantaneous value.  For forces that don't change over time, that's fine.
If a force is changing over time (or might be changing over time), you might not have sufficient information to compute the value at any one instance in time.  But you might have enough to compute the average value.
If a ball is sitting on a scale, the situation is static and you can calculate the force (the instantaneous force) for the ball on the scale.
If the ball is bouncing on the scale, you can't report "the force", since it changes as the ball interacts with the scale and then separates from the scale.  But you could calculate an average force over time (which would be the same as the first force if losses are ignored).
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