What is your explanation why the block stopped and did not begin sliding again?

Question

wooden block is sitting on an inclined plane near the bottom. The student gave the block a flick and it moved up the inclined plane then stopped. It is expected that the block to start sliding back down the inclined plane but it did not. It just sat there near the top.

Answer 1

Hi Alone O.,

You could postulate that the force of "gravity" varies along the inclined plane. This might be reasonable if the experiment takes place in a rotating frame of reference in outer space somewhere (but you're unaware of that condition). Even so, you'd have to jump through hoops (so to speak, always more difficult literally when you're floating in space!) to justify the block just stopping.

So if I were you, I might invoke the general principle that coefficient of sliding friction is less than the coefficient of static friction. I leave the details to you. But if you're going to get the maximum utility from your physics course, you should ask yourself why that is, at a microscopic/mechanistic level. Who knows, you might come up with some additional possibilites for the inclined plane/block interface, such as:

1) texturing such that sliding friction is different in the two directions (as used in waxless skis!)

2) exhaustion of a liquid lubricant, or

3) change in atmospheric pressure such that a lubricant becomes ineffective at coincidentally exactly the pausing point (such as graphite does in a vacuum)

4) ??

Myself, I think a variant of this scenario might explain why the pyramids happen to have "stopped" exactly where they did in the Egyptian desert. Stranger ideas about the pyramids have been advanced recently, Google "Carson" and "pyramids" to check.

-- Cheers, -- Mr. d.