How Does It Work

According to Newton’s First Law of Motion, objects in motion tend to remain in motion unless acted upon by an external force. In this case, Newton’s Law requires the water to continue moving along a tangent to the circle. Thus, a force is required to keep it always turning toward the center of the circle. Even when the water is at the top of the circle, the force continues to accelerate the water downwards in the circular pattern, keeping it from spilling down on your head.

But what about gravity pulling the water downwards when it is at the top of the arc? One can explain that Force = Mass x Acceleration.  The force of the circular motion is greater than the force of gravity on the water, as long as the acceleration remains high enough. Therefore, the water doesn’t leave the bucket or glasses.