Spring Force Help Needed!

1. Dec 17, 2005

avb203796

When a 80.0kg mass is placed (not dropped) on topof a spring, the spring is depressed by 20 cm. The same spring is then used to launch a 2.0 kg object vertically. If the spring is compressed 5.0 m by pushing it down, and the object is then placed on top and the spring released, what will be the launch velocity of the 2.0kg object? Draw and label the force diagram as part of your answer.

I do not really ge twhat I am supposed to do here and I have no clue where to start. Any help would be greatly appreciated!

2. Dec 17, 2005

andrewchang

try using energy to solve for the spring constant.

3. Dec 17, 2005

avb203796

But how would I find the energy?

4. Dec 17, 2005

andrewchang

$$mgh = \frac {1}{2} kx^2$$

5. Dec 17, 2005

avb203796

But what would be my h and what would be my x?

6. Dec 17, 2005

avb203796

I am not given the h or the x am I?

7. Dec 17, 2005

daniel_i_l

The x is the amount that the spring is compressed. In this case the h is the same as the x, because the only change in the masses hight is because off the compression of the spring.

8. Dec 17, 2005

avb203796

Ok thanks!

9. Dec 17, 2005

avb203796

Wait, am I using the 8o.o kg and the 20cm or the 1.0kg and the 5.0m?

10. Dec 17, 2005

daniel_i_l

80.0kg and the 20cm

11. Dec 17, 2005

avb203796

so then I get 68.6 for k but then what do i do with that?

12. Dec 17, 2005

kp

Force = kx
F/x = k

what is the Force...check your units.

13. Dec 17, 2005

avb203796

I am not sure what you mean?

14. Dec 17, 2005

andrewchang

once you find the spring constant, you can use it for the second part of the problem.

15. Dec 18, 2005

daniel_i_l

Hint: Conservation of mechanical energy.