# Homework Help: Looking For Help with Catapult Project

1. Mar 31, 2015

### Harrison Gillis

For my term project we were challenged to build a catapult that can be adjusted to hit 3 pre-determined targets, and 3 surprise targets. I have come up with a design, but I'm not sure it's the most effiecient materials wise. I also would like some insight on the design.

I believe I should stop the arm at 45 degrees as that will make it easier when I have to calculate the force needed to hit the target.

Last edited by a moderator: Mar 31, 2015
2. Mar 31, 2015

### Staff: Mentor

Welcome to PF!

Your drawings are nice but you haven't shown the actual mechanism that does the work of catapulting
like a bungee cord or a weight or something else.

Have you looked online for examples?

3. Mar 31, 2015

### CWatters

Have a think about the factors that control how far the missile will go.

Will it be easier to adjust the force of the spring (?) or the launch angle?

4. Mar 31, 2015

### Harrison Gillis

I was planning on using bungee cords. I have looked online but most are not adjustable. Some targets will be on the ground, some will be on a wall.

As for which would be easier I'm not sure. I could use a force spring to measure my Newtons but that would be an approximate force.

5. Mar 31, 2015

### Staff: Mentor

You could also use medical tubing too. It was popular with slingshots.

6. Mar 31, 2015

### Staff: Mentor

What is the projectile? How big are the targets?

I would probably make both the spring force and the stop angle adjustable, and then use lots of practice shots to calibrate both adjustments to give me the best repeatability.

Putting some targets on walls is an interesting twist. Have you thought about how best to handle those?

7. Mar 31, 2015

### Harrison Gillis

We get to choose our projectile, I'm thinking golf ball or marble. For the targets on walls, correct me if I'm wrong, but couldn't I use $h(t) = (1/2)at^2 + vt + h$ and find my two points at which h = target height and omit the incorrect one?

8. Apr 1, 2015

### andrevdh

A compression spring below the arm?