When I learned how to juggle, I certainly did not decipher the act for a major physics action but rather just learned on instinct. The physics behind juggling can actually be pretty impressive if you take a moment to look at it. Major concepts involved include parabolic arcs, speed, velocity, acceleration, air resistance and the force of gravity. In addition, the juggler needs to determine the objects center of gravity to effectively toss it in the air and predict its path. The most important force is gravity, because without it juggling obviously wouldn't be possible. As we know the gravitational acceleration is 9.81 m/s, and the force becomes the only one acting on the object the minute it leaves the jugglers hand. This needs to be taken into account, which is why jugglers throw their objects at different heights depending on how many of them their are. However high throws can become problematic, for the smallest error becomes incredibly noticeable at such heights. In addition one has to take into account the objects mass to know how much force to apply to it to make it have the proper amount of inertia for the act to be successful.