# Homework Help: Spring Collision

1. Nov 19, 2007

### grouchy

[SOLVED] Spring Collision

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

A 8 kg block is pushed by an external force against a spring with spring constant 173 N/m until the spring is compressed by 2.1 m from its uncompressed length (x = 0). The block rests on a horizontal plane that has a coefficient of kinetic friction of 0.56. The acceleration of gravity is 9.8 m/s^2. Remember: The block is not attached to the spring.

After all the external forces are removed (so the compressed spring releases the mass) how far D along the plane will the block move before coming to a stop? Answer in units of m.

The attempt at a solution...

Ui - Wf = 1/2mv^2

(0.56)mgd = 1/2kx^2 - 1/2mv^2 then I just moved everything over to get d but I used 1/2kx^2 to get the v by setting it equal to 1/2mv^2 so it just cancels out...

2. Nov 19, 2007

### PhanthomJay

But it is given that the block comes to a stop, so v_f =???

3. Nov 19, 2007

### grouchy

attempt 2...

u = coefficient of friction)

1/2kx^2 = umg*d

d = 1/2kx^2/umg
d = 8.668616071m (wrong again)

4. Nov 19, 2007

### PhanthomJay

Too many significant figures! Try 8.7 meters from its released position.

5. Nov 19, 2007

### grouchy

that doesnt matter thought, the site even says "do not use" sig figs....

6. Nov 19, 2007

### PhanthomJay

Well maybe the darn site is looking for the distance it travels from its original position before the spring was compressed, from x =0, in which case maybe it's looking for d = (8.7 -2.1) = 6.6m?

7. Nov 19, 2007

### Staff: Mentor

Your method is fine. I would check your calculation again (I get a different 3rd digit). It shouldn't matter, but that depends on how fussy the online system is.

More important than that is where are you supposed to measure Distance from? From the compressed position (x = -2.1 m)? Or the unstretched position (x = 0)? (Based on how you described it, I'd say from the compressed position, just like you found.)

8. Nov 19, 2007

### grouchy

yup. it was -2.1 so 6.6m. thx guys