Homework Help: Cross products- how to determine the angle?

1. Jan 11, 2012

wu7

Consider the following problem:
You strike a pool ball (radius R) at some height h above the table, so that it immediately rolls without slipping.
At what height h should you strike it?

I worked out the problem and got 1.4R, which is indeed the answer.

However, at one point I made the assumption that I0ω = Δp*(h-R)*sin90.
As in, the momentum is perpendicular to the radius, so the cross product is just times 1.

I don't understand why sin90 is acceptable. By that logic, you could hit the pool ball anywhere horizontally, and it would be perpendicular to some R. what R? In this case it seems to be the R which is vertical.

I thought you had to take the cross product at the point of contact- and use the radius which goes from the center to that point of contact. THEN cross that with the horizontal force.

I know this is the way you're supposed to do it with a rotating stick- why not with a sphere?

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