# Calculate Friction Force + Coefficient

1. Oct 13, 2008

### halo9909

1. The problem statement, all variables and given/known data
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?

2. Relevant equations

I am not relly sure of the equations used..

3. 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

2. Oct 13, 2008

### nicksauce

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?

3. Oct 13, 2008

### halo9909

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. Oct 13, 2008

### nicksauce

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. Oct 13, 2008

### halo9909

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. Oct 13, 2008

### nicksauce

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. Oct 13, 2008

### halo9909

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

8. Oct 13, 2008

### nicksauce

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. Oct 13, 2008

### halo9909

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. Oct 13, 2008

### nicksauce

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

11. Oct 13, 2008

### halo9909

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. Oct 13, 2008

### nicksauce

How does the force of friction relate to the normal force? What is the value of the normal force?

13. Oct 13, 2008

### halo9909

O i see, its more clearer when someone else tells it
so the 5N/49N making it .102
Resolved!