Catapults take a number forms. Some fire using elastic, some use the elasticity of coiled rope, and some use a counterweight. The latter is by far the easiest to investigate for a physics project. Even better, you can choose from many plans and instructions for building this kind of catapult for free online.
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The easiest investigation you can do on catapults is to alter the counterweight and investigate how this affects the distance you can fire a projectile. Build a catapult according to a plan from the Internet (see Resources). Change the weight of the counterweight in small, steady increments. Measure the distance the projectile fires with each increase. Do a number of repeated trials to ensure accuracy.
Angle of Descent
Another concept to investigate is how the path of the counterweight's descent influences its projectile-firing power. Make an extension for the arm that holds the counterweight. The extension should be modifiable so that it can stick directly out in line with the firing arm, attach perpendicular and be set at a range of angles in between. Use the different angles but maintain the same weight throughout the experiment to investigate how the position of the counterweight affects the steepness of its path of descent, and how this affects the firepower.
In the medieval era, when catapults and siege engines of all kinds came into their own, engineers found they couldn't fire heavy catapults from the walls of their castles because the vibration caused the wall to be shaken apart. Investigate vibration by firing the catapult from a wooden table. Place a coin at each corner of the catapult and draw a pencil line around it. Fire the catapult and note how far the coin is shaken from the circle. Next, lay the catapult on sheets of various materials and investigate whether the amount of vibration decreases.
Extend the previous investigation by looking at what effect the dampening of vibration has on the efficiency of the catapult. Record the distances the catapult fires its projectiles, using each of the different dampening materials. Also, record the size of the group that each projectile makes when it lands when using the different materials as a dampener. Determine whether better dampening improves the range and consistency of the shots.
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