Re: [Phys-l] Ball on an inclined plane

["R. McDermott" <rmcder@gmail.com>]

I agree with the analysis, and note that mg is approximately 10N.  Why would 
we (or the problem's creator) want to "divide out mg"?  The "solution" 
provided by the book seems to be unitless?!  Was the mass supposed to be 
0.1Kg?

----- Original Message ----- 
From: "Josh Gates-fac" <Josh_Gates-fac@nobles.edu>
To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
Sent: Wednesday, August 02, 2006 6:37 PM
Subject: Re: [Phys-l] Ball on an inclined plane


> here goes:
> What I'm getting from the problem description is that there's a vertical
> wall (WLOG, put this on the left) and an inclined plane going from lower
> left to upper right, theta above horizontal.  The forces on the ball are:
> gravity - mg down
> force of wall - directed horiz. to right
> force of inclined plane - directed normal to plane, towards upper left of
> diagram.
>
> net force on ball vanishes in all directions.
> In particular, choosing right and up as positive,
> Fnet, x = Fwall - Fplane sintheta = 0
> Fnet, y = Fplane costheta - mg = 0
>
> from this,
>
> Fplane = mg/cos theta
> and
> Fwall = mg tantheta
>
> dividing out mg gives Feynman's solution.
> jg
>
>
>
> Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu> on
> Wednesday, August 02, 2006 at 4:49 PM -0500 wrote:
> > 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
> >
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>
>
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