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Calculate Friction Force + Coefficient

  • Thread starter halo9909
  • Start date
  • #1
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Homework Statement


A force of 30 N accelerates a 5.0-kg block at 5.0 m/s2 along a horizontal surface.

How large is the frictional force?
(b) What is the coefficient of friction?


Homework Equations



I am not relly sure of the equations used..

The Attempt at a Solution



Well first i calculated the F Net which would be the mass * 9.8 in this case it was 5.0kg * 9.8 which is 49N, then i did 30N/49N which was .6122 apparently that is the the coefficient, and calculating (a) I have no idea
 

Answers and Replies

  • #2
nicksauce
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What are the things you know?

->The normal force is mg=49N.
->The friction force is the coefficient of friction times the normal force, [itex]49\mu[/itex].
->From Newton's second law, m*a=30 - force of friction.

Can you solve the problem with this information?
 
  • #3
37
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So with 49N I would subract 30
49N-30N=19N
The force of friction would be 19N?

Then for the coefficient it would be 19/49? ~.3877?

Apparently I am still doing something wrong,
 
  • #4
nicksauce
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Don't ignore the fact that the block is accelerating! F=ma ! The force on the block is the Pulling force minus the friction force. ma is 5kg * 5m/s^2.

On a different note, did you have some sort of external confirmation that the coefficient of friction is .6122? I do not get the same answer.
 
  • #5
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Don't ignore the fact that the block is accelerating! F=ma ! The force on the block is the Pulling force minus the friction force. ma is 5kg * 5m/s^2.

On a different note, did you have some sort of external confirmation that the coefficient of friction is .6122? I do not get the same answer.
well since this is a webassign and we have 50 chances to submit the correct answer, i been trying but somehow still hasnt turned up with the correct value.

Would you have to consider the acceleration when calulating the friction force? I thought it was extra information
 
  • #6
nicksauce
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You must use F = ma. F = Pulling force - friction force. ma = 5kg * 5m/s^2. There is no other way to find the force of friction.

The only other information you have is that normal force = 49N, and friction force = coefficient of friction * 49N. This information alone cannot determine the force of friction or the coefficient of friction.
 
  • #7
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So your saying this problem cannot be solved?
doesnt seem likely
 
  • #8
nicksauce
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The problem can be solved. As I have said before, you have to use F = ma = Pulling force - Friction force. You know m, you know a, you know the pulling force, and you don't know the friction force. One equation, one unknown, can be solved.

Friction force = Pulling force - ma.
 
  • #9
37
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hmm
49N is the Force
ma = 25N
so 49N-25N=24N

so the friction force should be 24N but it is saying it is wrong
 
  • #10
nicksauce
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As stated in the problem, the pulling force is 30N. 49N is the weight / normal force.
 
  • #11
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now i got it, the friciton force = 5N which was the pulling force =30N - (mass5 * acceleration5)
=30 -25 so it is 5N

now to find the coefficient of friction
 
  • #12
nicksauce
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How does the force of friction relate to the normal force? What is the value of the normal force?
 
  • #13
37
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O i see, its more clearer when someone else tells it
so the 5N/49N making it .102
Resolved!
 

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