# Spring Elastic Potential Energy! Help!

1. Nov 16, 2009

### kmag

1) A slingshot consists of a light leather cup, containing a stone, that is pulled back against two parallel rubber bands. It takes a force of 11 N to stretch either one of these bands 1.1 cm.

(a) What is the potential energy stored in the two bands together when a stone is placed in the cup and pulled back 0.19 m from the equilibrium position?

(b) With what speed does the stone leave the slingshot?

2) A child's toy consists of a piece of plastic attached to a spring in the following figure. The spring is compressed against the floor a distance of 2.05 cm, and the toy is released. If the toy has a mass of and rises to a maximum height of 57.0 cm, estimate the force constant of the spring.

Note: the height of the toy in a compressed state is negligible in comparison with the maximum height.

Round your answer to three significant figures. Take the free fall acceleration to be .
I've already tried using the equations F= kx and Us=1/2kx^2 but I'm not getting the right answer. I filled F in with 11 N and x with 0.011 m to find k, I got 1000. Then I plugged 1000 into the other equation and got 1/2(1000)(0.19^2). I got 18.05 and rounded to two sig figs 18 but it is still not right. What am I doing wrong? Please help!

2. Nov 16, 2009

### Staff: Mentor

Welcome to the PF. You need to show us the relevant equations and show us your attempt at a solution before we can be of much tutorial help.

What are the main equations that we use for spring questions? The first relates the force F to the sprink constant k and the distance the spring is pushed/pulled from equilibrium.

The second relates the energy E in a pushed/pulled spring to its spring constant and the distance it is pushed/pulled.