# Fun with Springs

1. Oct 30, 2006

### the_d

:rofl: How do i find the velocity of a block of mass 100 lb being dropped a distance 5 ft onto a spring whose stifness is 100 lbs/ft after the spring has deformed 4 in?

im sure it has somthing to do with work of a spring force but i dont know exactly how. any ideas???

2. Oct 30, 2006

### jamesrc

Can you write equations for the total energy of the mass and spring (kinetic and potential) before being dropped and at the moment the spring is deformed 4"? If you can, then you are well on your way to solving the problem.

3. Oct 30, 2006

### the_d

ok ill see what i can do, thanx

4. Oct 30, 2006

### the_d

do i use T1 + U = T2???

5. Oct 30, 2006

### Dorothy Weglend

How does the work done by gravity relate to the work done by the spring force?

6. Oct 30, 2006

### the_d

its in the opposite direction

7. Oct 30, 2006

### Dorothy Weglend

What do they sum to?

8. Oct 30, 2006

### the_d

zero.....................

9. Oct 31, 2006

### Dorothy Weglend

Well, there you go then. Problem solved.

10. Oct 31, 2006

### Staff: Mentor

No reason to think that the work done by the spring force will equal the work done by gravity. Note that it doesn't say that 4 in is the maximum compression of the spring... the block keeps going!

Instead, consider conservation of energy (assume no energy is lost when the falling block hits the "massless" spring). What's the initial energy of the block just as its being dropped? What's the final energy of the block + spring system when the spring has been depressed 4 in?

11. Oct 31, 2006

### the_d

good explanation, i think i got it now thanx 4 help