Hooke's Law

[premier]

I'm having problem with the constant. My answer differs from that of the book. Maybe someone can point out my mistake. This is the problem.

A spring has a natural length of 2ft and a force of 15lb is required to hold it compressed at a length of 18in. How much work is done in stretching this spring from its natural length to a length of 3ft?

These are the steps I followed.

Displacement
2ft x = 0
3ft x = +1
18" x = - .75

Hooke's law, where K is the constant, x is the the deformation of the elastic body subjected to the force F

F(x) = kx
-15lb = -.75K
20 = K

My constant, K, is 20 whereas in the answer book it says that K is 30. I don't understand how it could be 30. Thanks for the help.

 

[askmemath]

Displacement
2ft x = 0
3ft x = +1
18" x = - .75
[\code]
18" =1.5 feet

=>x= -.5

I hope you can take it from here.[/quote]

 

[premier]

Thanks a lot. That really helped