Question

Suppose a table tennis ball is immersed in a fluid and held down by a string. The container is placed on a scales. What will happen to the reading of the scales if the string breaks? How can you correctly explain this using Newton's Third Law?

I am assuming when the string is cut, the ball experiences only its weight and buoyancy. If buoyancy exceeds the weight, the ball will experience a net force upwards (Fb - W). So, according to Newton's Third Law (every action has an equal and opposite reaction), will the container experience the same magnitude of the force (Fb - W) but act downwards? If so, will the reading on the scales be greater?

Answer 1

The scale's reading will momentarily increase when the ball is released from the string, as the buoyant force that propels the ball upward applies an equal and opposite force to the fluid, which is transmitted to the scale.

Step-by-step explanation:

When the table tennis ball, submerged in fluid and held down by a string, is released from the string, the reading on the scales will momentarily increase. This is explained by Newton's Third Law of motion and the concept of buoyancy. Initially, the ball is exerting a force on the fluid due to its weight and the string is holding it in place, providing an opposite force, which is transmitted to the scale through the fluid. When the string is cut, the buoyant force, which is equal to the weight of the displaced fluid, acts on the ball making it accelerate upwards. According to Newton's Third Law, the ball pushes downward on the fluid with an equal and opposite force. This additional force on the fluid is transmitted to the scale as well, temporarily increasing the reading. Eventually, the ball will buoy up and leave the fluid, resulting in the scale reading returning to the weight of just the fluid.