# Dynamics angular speed help

1. Mar 26, 2012

### zsp92

A billiard ball is cued by striking it horizontally at a distance d=10mm above the center of the ball. The ball has mass m=0.2kg and radius r=30mm. Immediately after the strike, the center-of-mass of the ball moves with linear speed v=1m/s. Find the angular speed of the ball immediately after the strike. Ignore friction between the ball and the table during the strike.

I need help solving this problem since i dont even know where to start. Even an initial pointer would help me loads.

2. Mar 26, 2012

### K^2

Realistically, the amount of English will depend on friction between cue and billiard, but since no useful information is given here, I think they want you to assume that the impact force is applied horizontally, which isn't a bad estimate, to be fair.

What you need to keep in mind is that while the impact force varies, the ratio of force applied to torque remains fixed at τ(t)=d*F(t). Now, you know that dp/dt = F(t) and dL/dt = τ(t) = d*F(t). So while you have no idea what the actual force profile F(t) is, you can still say that ΔL = d*ΔL. You know what Δp is from velocity and mass, so you can get ΔL. Knowing ΔL, you should be able to work out the angular momentum by looking up the moment of inertia for a solid sphere.

3. Mar 27, 2012

### Staff: Mentor

Thread locked at the request of the OP.