# Orientation on Calculus Work Problem. Hooke's Law.

1. Aug 5, 2013

### Dan350

A force of 16,000lb compresses a string from its natural length of 13 inch to 8 inch. Find the work done to compress it to the first inch

W=$$\int F dx$$

F=kx
16000=K(5)
3200=K

W=$$\int F dx$$

W=$$\int_1^{13}\!\ 3200xdx$$

[1600x^2] from 1 to 13

w= 268800ftlb

Am I right?
I think the trick here is the limits.
If they would be from 0 to 1,, aint that to little work?
I mean, as you compress down, it's harder to do it. The work has to increase as you reach 0 inch
Am I right?
If not, please explain

Or is it from 12 to 13?

how I viewed is that they want to know the work as you compress it to the first inch.. in this case 1

I need a little orientation

| 16000Lb |
/----*------------------------*----------------------/
0inch 1inch 8inch 13inc
Thanks!

Last edited: Aug 5, 2013
2. Aug 5, 2013

### haruspex

Wrong units for a force.
Extra 0 crept in. What units do you want the work in? What units is the 5 in?
What are the initial and final lengths of the spring as it goes through "the first inch" of its compression from 13 inches to 8 inches?

3. Aug 5, 2013

### Dan350

force is 16000lb
using hooke's law I got the K
16000lb=K(5) 5 as the inches that the Force compressed

The lenght of the string is 13 inches
It was compressed 5 inches by a 16000lb force
I need to find the work of the first inch

Thanks

4. Aug 5, 2013

### haruspex

A spring, presumably, not a string.
So answer my question:
"What are the initial and final lengths of the spring as it goes through "the first inch" of its compression from 13 inches to 8 inches? "​

5. Aug 5, 2013

### Dan350

1 and 13
the spring is 13 inch long. from 0 to 13 there's no work, now, whether it compresses or streches, a work can be calculated

6. Aug 5, 2013

### haruspex

No.
The x in your integral is the extent of compression. The spring starts at its relaxed length of 13 inches. That's x = 0, no compression. It is compressed from 13 inches to 8 inches. That's a compression of 5 inches, x = 5. What is the value of x when the spring has been compressed by only the first of those five inches of compression?