# 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.

## Answers and Replies

Stephen Tashi
Science Advisor
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
Science Advisor
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
Science Advisor
OK, so then I know that t=3, and I(impulse)=20, If F is unknown then F=I/T= 6.66N

Yes

keithcuda
Thank you for the help. I greatly appreciate it :)