# Need help with force of spring needed to launch mass from catapult

1. Jan 25, 2013

### DannyD

Hi All

I need help with a physics calculation.

I have a mass of 3kg that has to be launched for earth (9.81m/s^2 gravitational acceleration) at an angle of 30 degrees from horizontal from a catapult. The distance the mass can travel in the catapult is 2m.

What I need to know is what is the spring constant that the spring needs to have in order for the mass to leave the catapult at 20m/s?

Kind regards
Danny

2. Jan 25, 2013

### szimmy

Are you familiar with the formulas of energy?

3. Jan 25, 2013

### DannyD

Yes I am. I'm currently studying mechanical engineering, my brain is just still in holiday mode, if I can be directed in the right direction that would also help alot.

Regards

4. Jan 25, 2013

### szimmy

Spring potential initial = Kinetic final

5. Jan 26, 2013

### DannyD

Cool, thanks!

6. Jan 26, 2013

### PhanthomJay

But don't forget that the mass is launched at an angle, not horizontally, so that in addition to the KE change, the gravitational PE of the mass also changes, during its motion along the catapult.