# Potential energy

1. Nov 5, 2009

### matt_crouch

1. The problem statement, all variables and given/known data

i wasnt really sure where to put this question. its kind of a maths question i suppose

the potential energy funtion u along the x-axis is given as

U=-U0x2

the question is really why do i not use the product rule on this equation as it is two variable multiplied together

i am told that the answer is u02x
but im a bit confused as to why you dont seem to do anythign with the first term apart from multiplied by -1 but still remains there. Is it to be treated as a constant? if so why does it not = 0 when differentiated.

any help would be appreciated =]

2. Relevant equations

3. The attempt at a solution

2. Nov 5, 2009

### Staff: Mentor

I presume the question is to find the force given that potential function. In any case, U0 is a constant. Go ahead and use the product rule if you like; the derivative of a constant is zero.

If y = ax2, what's dy/dx? (a is a constant)

3. Nov 5, 2009

### matt_crouch

sorry ye the question to find the force. and y=ax2 will differentiate to 2x but i dont understand in the question the answer has the constant still in it after its be differentiated

:/

4. Nov 5, 2009

### matt_crouch

aaahh wait i see if i differentiate it using the product rule i do get that answer..
i see why now

dy/dx= da/dx.x2+2x.a but because da/dx = 0 it removes the whole of the first term.

makes more sense now
cheers