The Great Ball Contraption is a series of Lego machines built to move marble-sized balls from one machine to the next. These machines, called modules, are constructed to a set of standards so that any module can feed balls into any other. Many builders will bring their modules to Lego fan conventions, like Brickfair, and arrange them together in a loop for continuous circulation of balls. https://www.youtube.com/watch?v=HvJg3YoehYk&t=1610s I've build many of these modules, but my most complex one to date appears at 10:08 in the video. It works by dropping the balls one at a time into a barrel, where a small electric motor pulls down a spring loaded arm. Once released, the arm accelerates the ball upward and around the underside of the arch, eventually landing on the other side. I designed this module to be overly-complex and entertaining as possible. As a result, there are many different kinds of energy being converted into one another as the ball travels from one end to the other. Almost all modules are powered by an electric motor, and all modules must lift the balls to a certain height to dump them into the next module, so the simplest module would use a motor to convert electrical energy from an outlet into kinetic energy of the spinning motor, and then use a simple lift or conveyor to transfer some of that kinetic energy to the balls in the form of gravitational potential energy. However, this module first takes the electrical energy from the wall and converts it into kinetic energy of the motor, but then converts it into elastic potential energy by compressing a spring. when the spring is released when in contact with the ball, it transfers some of its elastic potential energy into the ball's kinetic energy. As the ball travels up the arch, some of its kinetic energy is converted into gravitational potential energy, only to be converted back into kinetic energy as it continues down to the other side. However, the ball does gain a small amount of gravitational potential energy through this process, as it leaves the module slightly higher than it enters. The conversion from kinetic energy of the motor to elastic potential energy and then back into the ball's kinetic energy is crucial to getting the ball up to such a high velocity to travel on the underside of the arch. In order for the ball to achieve such a high velocity as it exits the barrel, it must experience a very large force and therefore a high acceleration. The motor itself is incapable of exerting such a large force that would accelerate the ball to such high speeds. With a gear reduction, the motor can exert a large torque and therefore a large force at the cost of speed The spring acts as an intermediate step. The motor exerts a comparatively small force on the spring to compress this, but does it over a relatively long time. However, when the spring is released, it exerts a large force on the ball over a short period of time. The impulse that the motor applies to the spring and the impulse the spring applies to the ball are the same, however, the spring is able to reach high enough speeds to change the ball's momentum enough to make it up and around the arch. The spring itself is fixed to a pivoting arm which forms a class II lever. The spring exerts a large force on the lever arm but is connected at a small radius from the pivot point, creating a torque. To compress the spring, a rotating arm attached to the motor exerts a force on the spring arm, but at a larger radius from the pivot than the spring. This means that the motor exerts a smaller force to create a torque greater than that created by the spring, at the cost of having to exert this torque over a longer distance. Once traveling out of the barrel, the balls undergo circular motion as they pass through the underside of the arch. There is no wall or guard underneath the balls. The inside of the arch creates a centripetal force which keeps the balls in a circular path, similar to the tension force created by swinging a ball in a circle from a string. The faster the balls travel, the greater this force is, so the balls must be traveling at a high velocity to overcome the force of gravity.
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Zach Pabisis a high school junior writing about his adventures in physics. Archives
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