Work done by the spring on an object

[DrHughes]

20) An ideal spring is hung vertically from the ceiling. When a mass 2.0 kg hangs at rest from it, the spring is extended 6.0 cm from its relaxed length. A downward external force is now applied to the mass to extend the spring an additional 10cm. While the spring is being extended by the force, the work done by the spring is:

a) – 4.2 J
b) – 3.6 J
c) – 3.4 x 10-5 J
d) 3.6 J
e) 4.2 J

I know formulas like Fsp = 1/2 kx^2 etc .etc.
I still don't know how to put them to use in this context

 

[Dick]

The formula you wrote is for the potential energy of the spring - not the force. This is not a good start. You'll have to make a better start before people can begin to help you.

 

[daniel_i_l]

If you want you can use energy to solve this question. Do you know how work and the change of potential energy are related?

 

[Doc Al]

I know formulas like Fsp = 1/2 kx^2 etc .etc.
I still don't know how to put them to use in this context

There's nothing wrong with using this formula for spring PE, if you know what it means. (Check your text!)

You can also attack the problem directly by considering the definition of work.