Electricity and SHM

1. Apr 5, 2015

erisedk

1. The problem statement, all variables and given/known data
In the figure mA = mB = 1kg . Block A is neutral while qB = -1C, sizes of A and B are negligible. B is released from rest at a distance 1.8m from A . Initially springs is neither compressed nor elongated.
View the figure above.
Equilibrium position of the combined mass is at x = ....... m

Note: Collision between A and B is perfectly inelastic.

2. Relevant equations

3. The attempt at a solution
a= 10m/s (of block B using qE/m)
v = root(2as) = 6m/s
mv = 2mv'
v' = 3m/s (combined velocity of the system)

1/2 mv^2 = 1/2 kx^2 - qEx
9 = 9x^2 - 10x
x = -0.588
The answer is -5/9, which is quite close to my answer but not exactly.
Can someone just tell me if what I've done is right?

2. Apr 5, 2015

rude man

Your equation makes no sense. It is not even dimensionally correct. qE is a force, the other terms are energy.
At equilibrium the electrostatic force must be equal and opposite to the spring force. That gives x = -5/9 exactly.

3. Apr 5, 2015

Brian T

He put qEx, not qE, which is an attempt to add in a work contribution i. E. Fd.

Although your method seems correct, just didn't want to give him the wrong idea

4. Apr 5, 2015

rude man

OK, I thought he meant E-sub-x. Still doesn't make sense, the equation I mean.
Well, my method has to be correct. May be some other way is too.

5. Apr 5, 2015

erisedk

Okay thanks, got it!