Originally Posted By: paul




according to the above free body diagram the block should fly upwards off of the incline , because the brunt of the
forces are in that direction.


Yes, if you make up values for the forces. Work it out correctly and it'll tell you what actually happens. If F3 is zero, the diagram tells you it acceleartes down the slope, and how fast. If you pick another value F3 it tells you which direction it'll go and with what acceleration. If you want to hold it stationary the diagram tells you what value of F3 to use for that too. F1 and F2 are determined by the block's weight.


Quote:

therefore a FBD is useless when trying to
calculate forces in nature as the ball would
apply a force to the table , like the block
above would provide a force downward opposite of F3,

It's useless when you're trying to fool people. You can't easily hide force components with a free-body diagram. More relevent here, you can't easily hide them _from yourself_. If you do the diagram you'll discover your device is very different from what you've described.

I know it's a simple idea. That's why it's so powerful. It's often easy to misjudge how forces work, to mix up reactions with actions, to reverse directions, to omit force components, to guess wrong values for magnitudes, etc. But it's hard to make a mistake with a free body diagram.

By the way, it's not always trivially easy to do. The block picture above requires some trig to calculate F2. You do actually have to do maths sometimes!

Last edited by kallog; 07/13/10 06:49 PM.