Purpose: This lab will examine the principles of energy transfer from potential to kinetic energy. To do so we will make catapults to determine if there is a difference in the amount of kinetic energy given to a projectile based on the storage of potential energy within the catapult.
Catapults are defined as any device used for hurling objects. This can be as simple as bending the spoon in the cafeteria to fling a pea at a classmate, or as complex as the medieval weapons that we see in movies, or even the slingshot device on the game “Angry Birds”. Regardless of how simple or complex the catapult, they function on the simple premise of turning stored potential energy into kinetic energy. The means by which the transfer of this energy from potential of the catapult to the kinetic energy of the projectile takes place is based on the design of the catapult. In general there are three distinct catapult structures: the ballista, the mangonel, and the trebuchet.
Regardless of the design, the means for developing potential to be stored is by displacing the projectile cup (the part of the catapult that will hold the projectile) from the resting position. This movement is done against ever increasing tension and compression forces within the arm of the catapult, or torsion within junction between the arm and the support of the catapult. The speed and distance by which the arm of the catapult is being moved from the resting position will determine how much potential energy is generated for storage within the catapult. Once released from the position, the arm and anything (such as a projectile) on the arm will undergo a transfer of energy from potential to kinetic energy. As the arm completes the motion back to the resting position, it hits a critical angle at which the projectile will no longer be traveling on the same path (since it is not directly connected to the arm) and will thus be launched.
The physics of all the mechanisms at play are fairly complex. That can be summarized, as the velocity of the projectile at launch will be related to the weight of the projectile, the weight of the projectile cup of the arm of the catapult and the velocity of the arm of the catapult and at what angle the projectile was launched from the catapult. Meaning, the faster the arm travels, the lighter the projectile and projectile cup the faster the projectile will travel. The distance the projectile will travel is going to be related to that launch angle.