# Friction force without coefficient of friction?

1. Sep 18, 2014

### Aris Holmes

I have a problem need to be solved here. Someone told me that the friction force must always have the friction coefficient. Is it right or wrong? and we can not calculate the friction force without friction coefficient because this force is only calculated by the coefficient and the Normal force?
Is there any way to calculate friction force without coefficient because I think that there are many materials can not identify the specific coefficient?
The friction force I think is the force like Newton's Force. So can I calculate the friction force by the second fomula of Newton force: F= m x a????

2. Sep 18, 2014

### ActionPotential

Is it static or kinetic friction?

3. Sep 18, 2014

### Aris Holmes

I think it is static friction. But I am not sure about it. So could you mind explaining both of 2 cases

4. Sep 18, 2014

### A.T.

Yes, sure. If you know m, a, and friction is the only force acting, or you also know the other forces.

5. Sep 18, 2014

### Khashishi

The idea of a friction coefficient is just an approximation. Some surfaces don't behave this way. For example, velcro. The resistance to movement is very strong, but it doesn't scale linearly with normal force.

6. Sep 18, 2014

### Staff: Mentor

ActionPotential's question in post #2 is very pertinent. If it is static friction, then you don't need to know the coefficient of static friction as long as the object is not yet sliding. Consider a box on an inclined plane. From a force balance in the direction along the plane, the friction force is statically determinate, and you don't need to know the coefficient of static friction. However, you do need to know the coefficient of static friction if you want to find out the conditions under which the box will start to slide.

Chet

7. Sep 18, 2014

### A.T.

If it is static friction, then knowing the coefficient of static friction is almost useless to find the actual friction (except for the upper boundary). You have to know the other forces and m & a, to find the actual static friction. But if you know all this then the type of force and any coefficients are irrelevant.

8. Sep 18, 2014

### Staff: Mentor

Yes. This is what I was trying to say in my own way. Also we should mention that, for a box on a stationary slide, ma = 0.

Chet