1. Oct 16, 2007

### physicsgirlie26

A marble moves along the x-axis. The potential-energy function is shown in the figure attached.

At which of the labeled x-coordinates is the force on the marble zero?
-a and c
-b and d

I was thinking it was at a and c

Which of the labeled x-coordinates is a position of stable equilibrium?
-a
-b
-c
-d

I was thinking this was at b

Which of the labeled x-coordinates is a position of unstable equilibrium?
-a
-b
-c
-d

This one i thought was c

Are this answers correct? Can someone check them and let me know please? Thanks :)

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2. Oct 16, 2007

### physicsgirlie26

Can someone just let me know if the first one is right? I know the second one is B and that the thrid one is supposed to be d and not c but i want to make sure i am right about the first one. I think its A and C because that is were the KE is zero making force zero, right?

thanks!

3. Oct 17, 2007

### qspeechc

Now, remember F = -dU/dx where U is potential energy. So if we want the force to be zero, we want dU/dx = 0.

Now, what is an equilibrium point? It is a point where the particle feels no force (see above). An UNSTABLE equilibrium is one where, if the particle only moves very slightly to either direction, it will continue moving away from the equilibrium point. That should be enough for you to find the unstable equilibrium point.

The second one is correct.