Block moved by spring - Determine speed

[starfish794]

A 2.5 kg block at rest on a tabletop is attached to a horizontal spring having constant 19.6 N/m. The spring is initially unstretched. A constant 20 N horizontal force is applied to the object, causing the spring to stretch. Determine the speed of the block after it has moved 0.900 m from equilibrium if the surface between the block and tabletop is frictionless.

I tried using the formula v= square root of k/m*(A^2-x^2) and got 2.52 which is the wrong answer. The 20 N force must need to be included in the problem in some way but I can't figure out how.

 

[radou]

What exactly do you mean by '0.900 m from equilibrium'? Do you mean, the point at which the spring was unstreched?

 

[starfish794]

That's what I took it to mean. I think the wording in the whole problem is bad.

 

[radou]

That's what I took it to mean. I think the wording in the whole problem is bad.

Okay, here's a hint: use the fact that the work of the force along the unknown displacement (i.e. extension of the spring) equals the change of the spring's potential energy. You should be able to calculate the stretch of the spring from that equation. Further on, by knowing the initial displacement, use energy conservation.