# Find the average force of friction

## Homework Statement

A 5kg box is tossed across the floor at 4 m/s and slides to a stop in 3 seconds. What is the average force of friction?

## Homework Equations

I believe the answer is 6.66N, I found that because the problem was pulled from the book. But I am trying to understand how and why. What formula is used, so if I come across it again, I have a better understanding to figure it out.

## The Attempt at a Solution

Stephen Tashi
You can calculate the initial momentum of the box. The momentum of the box is zero when it stops moving. What formula relates force to a change in momentum?

You can calculate the initial momentum of the box. The momentum of the box is zero when it stops moving. What formula relates force to a change in momentum?
That would be impulse, f*t
and f=m*a
so the impulse would be, (m*a)*t
impulse=(5*(4/3))*3=20

Last edited:
Stephen Tashi
That would be impulse, f*t
and f=m*a
so the impulse would be, (m*a)*t
impulse=(5*(4/3))*3=20
That would be impulse, f*t
and f=m*a
so the impulse would be, (m*a)*t
impulse=(5*(4/3))*3=20

Yes, if you use 4/3 as average acceleration. However, to solve for average F in equation for impulse, you'd do better to keep F as an unknown instead of writting it as m*a. To solve for F using F = m*a , you could claim that average force = mass * average acceleration.

Yes, if you use 4/3 as average acceleration. However, to solve for average F in equation for impulse, you'd do better to keep F as an unknown instead of writting it as m*a. To solve for F using F = m*a , you could claim that average force = mass * average acceleration.
OK, so then I know that t=3, and I(impulse)=20, If F is unknown then F=I/T= 6.66N

Stephen Tashi