Homework Help: Calc- Force Problem

1. Sep 6, 2010

J.live

1. The problem statement, all variables and given/known data

A Force of 7 pounds is required to stretch a 40 inch spring to a length of 43 inches. How much work will be done in stretching the spring an additional 6 inches from its already extended length of 43 inches?

3. The attempt at a solution

I know how to integrate , just having problems with taking limits.Will I take the limit from 3 to 9 inches or is it 43 to 49 inches? Will be great if someone can clear it out by explaining why. Thank you.

2. Sep 6, 2010

LawlQuals

On second thought (this is edited), I will assume you understand how to find the spring constant $$k$$ and just were confused about the limits, which I should have done in the first place since that is what you asked.

Can you clarify how you are setting up your expression for calculating the work done? The limits of integration have all to do on just how you *choose* to write the spring force equation, nothing else.

3. Sep 6, 2010

J.live

Yes i Have calculated the spring constant which is

F(x)= k*x

F(9) = 7k

k= 9/7

Where I get stuck is taking limits for k.

4. Sep 7, 2010

nobody? :/

5. Sep 7, 2010

Dick

In the spring constant equation F(x)=k*x, x is the difference between the stretched length of the spring and the unstretched length of the spring. Try and compute k again. You know F(3)=7.

6. Sep 7, 2010

J.live

So..

K= 7/3. Then I take limit from 3 to 9 inches?

7. Sep 7, 2010

Yes.

8. Sep 7, 2010

Thank you.