Help with elastic potential energy

[jbjohnybaker]

Homework Statement

A child's toy shoots a rubber dart of mass 7.8g, using a compressed spring with a force constant of 3.5 x 10^2 N/m. The spring is initially compressed 4.5cm. All the elastic potential energy is converted into kinetic energy of the dart.
What is the speed of the dart as it leaves the toy?

Homework Equations

Ee = Ek
Ee = 1/2 k x^2
Ek = 1/2 mv^2

The Attempt at a Solution

Ee = Ek
1/2 (3.5 x10^2)(?) = 1/2 (.0078)v^2 ??]

 

[method_man]

Homework Statement

A child's toy shoots a rubber dart of mass 7.8g, using a compressed spring with a force constant of 3.5 x 10^2 N/m. The spring is initially compressed 4.5cm. All the elastic potential energy is converted into kinetic energy of the dart.
What is the speed of the dart as it leaves the toy?

Homework Equations

Ee = Ek
Ee = 1/2 k x^2
Ek = 1/2 mv^2

The Attempt at a Solution

Ee = Ek
1/2 (3.5 x10^2)(?) = 1/2 (.0078)v^2 ??]

How much is the spring compressed?

http://www.physics247.com/physics-tutorial/spring-potential-energy.shtml"

 

[A_B]

equating Ee to Ek is indeed the right way to go. I suggest writing that equation in symbols (not yet plugging in the numbers) and then solving for v.
You'll get an expression for v in terms of k (spring constant), m (mass of the dart) and x (initial compression of the spring), all of which are given.
Then you simply plug in the given values and compute the value for v.

Alex