# Homework Help: A spring is compressed by 90 cm

1. Mar 4, 2012

### ScienceGeek24

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

A spring is compressed by 90 cm at the bottom of a frictionless hill of height 18m. the spring is realeased and propels a 2 kg block over the hill. At the top of the hill, the block's speed is 6 m/s. Find the force constant (spring constant) of the spring.

2. Relevant equations

1/2kx^2, 1/2mv^2 and mgy

3. The attempt at a solution
mgy=1/2kx^2+1/2mv^2 I solved for K and it did no give me the right answer. what i'm I doing wrong is this the right set up???

2. Mar 4, 2012

### spf

At the beginning, the whole energy of the system is in the elastic energy of the spring. When the block is at the top of the hill, the energy is in the potential energy of the block and the kinetic energy of the block. Thus, it's NOT the potential energy which equals the sum of the elastic energy and the kinetic energy as you have written BUT the elastic energy which equals the sum of the potential energy and the kinetic energy.

3. Mar 4, 2012

### ScienceGeek24

thanks man it worked!

4. Mar 4, 2012

5. Mar 4, 2012

### ScienceGeek24

But i forgot also to emphazie the second part of the question where is asking waht is the smallest compression of the spring that will allow the block to reach the op of the hill and not slide down?

I tried Fs=-kdeltax and it did not give me the right answer.

than i tried the same thing from part 1 solving for x and nope.

6. Mar 4, 2012

### spf

The only difference to part 1 of the question is that now the speed of the block at the top of the hill is not 6 m/s but... how much?

7. Mar 4, 2012

### ScienceGeek24

om/s yess!! thanks man!