Simple Harmonic Easy Qualitve problem, something I'm not getting

[Jediknight]

Homework Statement

Which of these forces could result in simple harmonic motion

F(x)=−9(x−7)3

F(x)=−9x+7

F(x)=−9x3

F(x)=−9x

F(x)=9x
F(x)=9x3

9x3 is 9x^3, same with itsopposite

Homework Equations

F=kx, the definition of shm

The Attempt at a Solution

I figured -9x and 9x were the only ones, i tried that

i tried those two and 9x+7 might work, just a measurement from a different spot

then I tried taking off -9x (leaving 9x and 9x+7) out of despirationI'm thinking maybe its a trick question and the answer might be all of them (like how pendulum at very small distances can be looked at as smh, or is there some fundamental concept I'm not getting here?

 

[berkeman]

Homework Statement

Which of these forces could result in simple harmonic motion

F(x)=−9(x−7)3

F(x)=−9x+7

F(x)=−9x3

F(x)=−9x

F(x)=9x
F(x)=9x3

9x3 is 9x^3, same with itsopposite

Homework Equations

F=kx, the definition of shm

The Attempt at a Solution

I figured -9x and 9x were the only ones, i tried that

i tried those two and 9x+7 might work, just a measurement from a different spot

then I tried taking off -9x (leaving 9x and 9x+7) out of despirationI'm thinking maybe its a trick question and the answer might be all of them (like how pendulum at very small distances can be looked at as smh, or is there some fundamental concept I'm not getting here?

Check the sign on your equation above that I've bolded in red. That's not the traditional definition used with Hooke's Law: http://en.wikipedia.org/wiki/Hooke's_law

 

[Jediknight]

good call, if F equaled kx a spring would push a load further and faster til it hit the speed of light!

(incase it wasnt obvious to everyone reading, F=-kx, F=-kx+c is fine to