Force analysis of a spring-bob system

[tellmesomething]

Homework Statement

A small bob of charge +Q and mass 5kg is suspended from a spring of force constant k=10^4N/m and is held at rest 50cm above an isolated point charge -Q fixed on the ground. The spring just stretched initially. When the bob is released, the naximun elongation of the spring is 10cm. Then Q=(25n)uc where n=?

Relevant Equations

None

I dont need anyone to do the sum directly please. I just need some hints, please dont give me the solution.

Im unable to understand how the bob will hit a maximum elongation, there are two forces accelerating the bob:
Gravitational force and electrostatic force.
There is one force deaccelerating it :
Spring Force

A max elongation means a point where the forces are balanced and the velocity of the bob equals 0

Would there be a point in time where the spring force becomes more than the downward force?

because I believe thats whats needed to make the velocity equal to 0 and then eventually the velocity vector would point in the upward direction

 

[TSny]

Im unable to understand how the bob will hit a maximum elongation, there are two forces accelerating the bob:
Gravitational force and electrostatic force.
There is one force deaccelerating it :
Spring Force

Yes

A max elongation means a point where the forces are balanced and the velocity of the bob equals 0

No. Think about the simple case of a ball thrown straight up in the air. At the highest point, v = 0. But, is the net force on the ball zero at this instant?

Would there be a point in time where the spring force becomes more than the downward force?

because I believe thats whats needed to make the velocity equal to 0 and then eventually the velocity vector would point in the upward direction

Yes. That's right.

Hint: Energy rather than force is your friend here.