Find the average force of friction

  • Thread starter keithcuda
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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

 

Answers and Replies

  • #2
Stephen Tashi
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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?
 
  • #3
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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
 
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  • #4
Stephen Tashi
Science Advisor
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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.
 
  • #5
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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
 
  • #6
Stephen Tashi
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OK, so then I know that t=3, and I(impulse)=20, If F is unknown then F=I/T= 6.66N
Yes
 
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Thank you for the help. I greatly appreciate it :)
 

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