Catapult building is a common competition in physics classes. If you have to build a catapult for your class, don't fall into the trap of thinking that more power will result in a better launch. More power behind your launch is definitely helpful but you'll have to use your physics knowledge to maximise the power behind your catapult. The most important thing to focus on is your catapult's trajectory. The right trajectory will be efficient and produce the results you're looking for.
Use the tension of bungee cords as opposed to springs to power your catapult. The cords should be attached to the front of the catapult and at the front of the catapult arm. When the arm is pulled back, the tension in the cord will pull the arm forward. This front-powered system will provide more power than rear-spring powered catapults.
Create an arm break that stops the arm at a 45-degree angle from the floor. A 45-degree angle is the best angle to maximise the efficiency of your catapult. The arm break can be made of the same material that comprises the frame of your catapult. It should run across the widths of the base and be in position to make contact with the catapult arm.
Measure the item you will be using for the catapult project. Next, find a cup to make your own holding device with the same dimensions. You want your projectile to fit snugly in the catapult to prevent the projectile from rattling around during the launch. The rattling around will prevent your projectile from launching at 45 degrees.
Weigh down the base of your catapult to prevent it from moving around during the launch. Any movement in the base can reduce the amount of power behind the launch and can also compromise the trajectory. It's particularly important to weigh down the back of the catapult since the natural movement of the arm with make the catapult lurch forward.