How Does the Force on a Particle Change with Position in a Potential Well?

[bakin]

Homework Statement

A particle is trapped in a potential well described by U(x)= 1.8 x²-b where U is in joules, x is in meters, and b = 4.5 J.

Find the force on the particle when it's at x = 2.8m
Find the force on the particle when it's at x = 0 m
Find the force on the particle when it's at x = -1.4 m.

Homework Equations

F(x) = -dU(x) / dx

The Attempt at a Solution

so I just differentiated U(x), slapped a negative sign, and ended up with F(x) = 3.6x

Plugged in 2.8m, ended up with -10.08, and it ain't right. Any ideas why? I'm thinking it might have to do with the phrase "potential well".

 

[bakin]

Never mind

F(x) = U(x) / x

edit: (never mind)² by using this, part B came out correct, (the answer would be zero), but it doesn't make sense. By plugging zero in there, you're dividing by zero, which is ILLEGAL. so i guess I am back here for help, lol.

edit: (never mind)³ /sigh the way i had it originally was correct, but online I put in the wrong number of significant figures. oh well.