[Phys-l] Ball on an inclined plane

[pschoch@nac.net]

I have just finished reading "Feynman's Tips on Physics" and found it very
enjoyable.  Then I got to the problems at the end of the book and the
first one intrigued me.

A Ball of Mass = 1 kg and radius of 3 cm is resting on an inclined plane
against a wall, the incline is at an angle alpha.  The question is to find
the force from the incline and the wall on the ball assuming negligable
friction.

Hubris is a terrible thing.  Thinking this was trivial, I plunged into the
problem and got nowhere near the solution in the text:

F(plane)= 1/cos(alpha)
F(wall) = tan(alpha).

I get (leaving mass unknown and g in the equations):

F(plane) = Mg cos(alpha)
F(wall) = 0.5Mg sin(2*alpha)

Any help in where I went awry would be appreciated.

Peter Schoch
Sussex CC