SUPER ULTRA MEGA CATAPULT [no joke]

[jp.mercado]

Homework Statement

Hello!, Beforehand i want to thank you for taking your time to look at this.
Here's the deal: There's a contest in my school for Science Fair.
We have to make a catapult and throw tennis balls as projectiles.
Here are the rules:
1) The catapult max measurements have to be 20 cm x 40 cm x 20 cm.
2) We have to get the biggest amount of tennis balls in 2 minutes in a trash can(diameter 28 cm, height 35 cm), which is 20 meters away from the catapult.
3) We can't use any bazooka or sling style.
4) NO explosives or combustibles.
5) We can use any type of structural material.

Homework Equations

Any ideas?

The Attempt at a Solution

None at the time of writing.

 

[lavalamp]

A stiff spring that you can compress, in a sturdy frame, to propel an arm that holds the tennis ball.

So basically a standard catapult.

Once you've calculated the forces and angles involved, this design should give you a repeatable way to fire the tennis balls at a specific angle at a specific speed. The trick will be getting it to not destroy itself when put into action.

 

[jp.mercado]

Any ideas of calculating the forces?
Any ideas of not destroying the catapult with the big forces?

 

[lavalamp]

Isn't the idea of a physics project to do it yourself?

I will say though, I'd use energy equations. Determine how much energy it takes to compress the spring, that is the energy that will be given to the throwing arm and the ball.

As far as making it not fall apart, you'd just have to construct it in a way that it's strong enough. The throwing arm will have a high force acting on it, as will whatever else the spring is attached to.