# Spring Force and block

1. Jul 1, 2009

### volraithe

A 1.54-kg block is held against a spring of force constant 1.47E+4 N/m, compressing it a distance of 0.100 m. How fast is the block moving after it is released and the spring pushes it away?

i thought u just use the formulas that F=kx and then plug that into F=ma, but i am not getting the right answer... anyone have any other suggestions as to what path I should take?
thank you.

2. Jul 1, 2009

### Pengwuino

Doing that will only give you the instantaneous acceleration. The problem with this is that the force will change continuously as the mass is pushed with the spring; this results in a constantly changing acceleration and we can't use our kinematics very easily. We want to use something else.

A very simple method for this problem would be to use the fact that the potential energy of the spring is $$\frac{{kx^2 }}{2}$$. We know that upon release, the spring is going to transfer all of it's energy into the mass. We of course know the energy of a mass as $$\frac{{mv^2 }}{2}$$. Equating both sides allows you to solve for the velocity of the mass.