# Calculating friction force

1. Sep 3, 2009

### semc

Hey guys i was told to calculate the frictional force of a ball bearing when it slide across a track. I was given the initial and final velocities , the distance between them and the weight of the ball. The 1st thing i though of was V^2 = u^2 + 2as and calculate for the deceleration then equate to F=ma. However i was told that i can find the frictional force by dividing the lost in kinetic energy by the distance travelled by the ball. I have not heard of this equation before can someone tell me what this is and where i can read more about it?

Another thing, why do we need to say that momentum is conserved in an isolated system? Cant we just say momentum is conserved without mentioning isolated system?

Last edited: Sep 3, 2009
2. Sep 3, 2009

### Fightfish

That'll be the concept of work. The frictional force does work (negative) on the ball bearing, which reduces its kinetic energy.

3. Sep 3, 2009

### semc

Oh.... damn should have though of that... What about isolated system? why do we need to specify inside an isolated system

4. Sep 3, 2009

### Fightfish

Consider the derivation of the conservation of linear momentum. If we pick a system such that there is a net external force acting on it - that force not being from something within our chosen system - then clearly our derivation does not hold and linear momentum is not conserved for this particular system.

5. Sep 3, 2009

### kuruman

You want the system to be isolated so that you can say that all the energy lost goes to heating by friction and not to anything else.