# Hooke's law sanity check

1. Nov 24, 2011

### bmxicle

1. The problem statement, all variables and given/known data
4 springs with a mass on each end are connected in series as below:

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m1
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m2
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m3
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m4

All the masses are mass m, the length of each spring is 1, and the spring constant is k, find the extension of each spring.

2. Relevant equations

f = ky

3. The attempt at a solution

So basically if i label each spring extension as $$y_1, \ y_2, \ y_3, \ y_4$$ then the only forces are the spring pulling from above, the spring pulling from below and the force of gravity on each mass. So for example for the second mass m2 from the top the equilibrium equation would be:

$$0 = ky_2 - ky_3 -mg$$

If this is right I can solve the rest of the problem easily, but I'm just getting thrown off by my use of newton's second law here.

2. Nov 24, 2011

### Villyer

0=ky2−ky3−mg

This seems to be an accurate statement.

Now how can you rewrite the ky2 and ky3s into the variables you do know, which are m, k and g?

3. Nov 25, 2011

### Redbelly98

Staff Emeritus
Yes, you are on the right track.

4. Nov 25, 2011

### bmxicle

Thanks, I can solve the rest of it pretty easily now. I think I was just over thinking which springs were pulling where.