# PhysicsMechanics- friction

#### Shah 72

##### Member
A snooker ball of mass 0.4kg is struck towards a cushion from 0.8m away with speed 3m/s. The surface of the snooker table has a coefficient of friction of 0.3. When the ball bounces from the cushion its speed is reduced by 20%. Find how far from the cushion it stops.

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#### Country Boy

##### Well-known member
MHB Math Helper
I have a basic problem with this. A snooker ball primarily rolls with only a slight amount of sliding. The coefficient of friction only applies to the sliding. To answer this you would have to know what part of the motion was sliding and I see no way to determine that.

#### Shah 72

##### Member
m=0.4kg, u=3m/s, coefficient of friction =0.3
By using Newtons law
F=m×a
-0.3×4=0.4a
a=-3m/s^2,
By using v^2=u^2+2as, v=2.05m/s when the ball hits the cushion
The speed decreases by 20%, so 2.05×0.8=1.64m/s. Again using v^2=u^2+2as, I get s=0.25m

#### Shah 72

##### Member
Oh sorry I had done a silly mistake. So initial velocity is 3 and when it hits the cushion, the final velocity is 2.05 m/s , this is reduced by 20% so now u= 1.64 m/s and v=0 ( as the ball stops) so the distance will be 0.448m

#### Shah 72

##### Member
I have a basic problem with this. A snooker ball primarily rolls with only a slight amount of sliding. The coefficient of friction only applies to the sliding. To answer this you would have to know what part of the motion was sliding and I see no way to determine that.
Thanks! I could work out the solution.

#### Country Boy

##### Well-known member
MHB Math Helper
I am glad. How did you handle the "rolling versus sliding" problem?

#### Shah 72

##### Member
I am glad. How did you handle the "rolling versus sliding" problem?
I just guessed rolling. Iam still in A levels so probably my textbook doesn't get into so much detail.

#### Country Boy

##### Well-known member
MHB Math Helper
If there were pure rolling, there would be NO friction!

#### Shah 72

##### Member
If there were pure rolling, there would be NO friction!
Yeah you are right.