# Calculate Friction Force + Coefficient

## 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

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nicksauce
Homework Helper
What are the things you know?

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

Can you solve the problem with this information?

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,

nicksauce
Homework Helper
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.

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

nicksauce
Homework Helper
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.

So your saying this problem cannot be solved?
doesnt seem likely

nicksauce
Homework Helper
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.

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

nicksauce
Homework Helper
As stated in the problem, the pulling force is 30N. 49N is the weight / normal force.

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

nicksauce