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Newton's Third Law explains that for every action, there is an equal and opposite reaction. In the context of our juggler, this law becomes vital to understand the interactions between the juggler and the bowling balls during juggling.
Whenever the juggler throws a ball into the air, the action of applying force on the ball results in an equal but opposite reaction of the ball exerting a force downward. This is the reason why, during the throw, the force on the scale decreases - some force now propels the ball upward, reducing the pressure on the scale.
Similarly, as the ball descends and the juggler catches it, the descending ball exerts an additional force downward. This causes the scale reading to increase temporarily. The juggler applies an upward force to decelerate and catch the ball, and the Third Law ensures the ball applies an equal downward force on the juggler, increasing the scale reading momentarily.

A bathroom scale is a device used to measure weight, which is the force exerted by an object under the influence of gravity. Weight is calculated as the mass of the object multiplied by the gravitational acceleration, generally approximated as \(9.81 \, m/s^2\).
When the juggler is holding all the bowling balls, the scale measures the sum of his and the balls’ weight - the combined gravitational force they exert downward on the scale.
As the juggler begins to throw or catch a ball, the weight measured by the scale fluctuates. When a ball is thrown, less of its gravitational force acts on the scale, leading to a momentary decrease in the recorded weight. Conversely, when catching, the added force increases the scale reading. These changes are temporary, and over a full juggling cycle, the average weight measurement remains stable.

Force analysis involves breaking down the forces acting on an object or system. For the juggler, multiple forces are at play, and understanding them helps in comprehending the scale reading changes.
When a ball is thrown, it experiences an upward force from the juggler while gravity continues to exert a downward force. The net force determines the motion of the ball. This upward force reduces the overall force the juggler exerts on the scale, resulting in a lower reading.
On the other hand, when the ball descends, gravity still acts on it, but the juggler must apply a stronger upward force to stop and catch it. Hence, the scale temporarily records a higher force.

• Throw Phase: Force from throwing reduces scale pressure.
• Catch Phase: Added force while catching increases scale reading.

The cycle of throwing and catching keeps the overall average force on the scale equivalent to the total weight initially when the juggler holds all three balls.