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?