Violation of Newton's third law

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Discussion Overview

The discussion revolves around the apparent violation of Newton's third law of motion when a stone is dropped into water. Participants explore the interactions between the stone, water, and gravitational forces, examining the implications of these interactions on the law's validity.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants assert that the stone sinks in water, which seems to contradict Newton's third law, as the water should exert an equal upward force on the stone.
  • Others clarify that acceleration is determined by the net force on each object, and the gravitational force acting on the stone is greater than the force exerted by the water.
  • A participant questions the nature of the forces involved, asking for clarification on what the upward force from the water should equal.
  • It is noted that the force of the stone on the water is equal and opposite to the force of the water on the stone, but the gravitational force on the stone is greater, leading to its downward acceleration.
  • Some participants emphasize that the forces involved in the interaction between the stone and the water are distinct from the gravitational forces between the stone and the Earth.
  • A viscous drag force is identified as the force exerted by the water on the stone, which also acts in the opposite direction when the stone displaces the water.
  • It is mentioned that air also exerts a force on falling objects, albeit smaller than that of water, and that this force can vary with velocity.
  • Another participant points out the presence of buoyant force acting on the stone, which is relevant to the discussion.

Areas of Agreement / Disagreement

Participants express differing views on the interpretation of forces involved in the scenario. While some clarify the application of Newton's third law, others maintain that the initial premise of violation remains unresolved.

Contextual Notes

The discussion includes various assumptions about force interactions and does not resolve the complexities of how different forces relate to one another in this context.

Madhusree Bera
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when a stone is dropped in a beaker containing water,it sinks.But according to Newton's third law of motion the water should also exert an equal force upwards...so the stone should float. This violating the third law but why?
 
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Acceleration is based on the net force on each object.

Newton's third law says that the force of one object on another is equal and opposite.

The gravitational force between the stone and the Earth is making the stone sink.

The water is not exerting an upward force on the stone equal to the force of gravity between the stone and the earth.
 
Madhusree Bera said:
the water should also exert an equal force upwards
Equal to what?

Madhusree Bera said:
This violating the third law but why?
If you think it does, you should explain why.
 
The force of the stone on the water, is equal and opposite the force of the water on the stone. Call these forces magnitude Fw
The force of gravity on the stone, call this Ws is greater than the force of the water on the stone Ws>Fw , Thus the stone continues to accelerate downward through the water.
The force of gravity on the Earth is equal and opposite the force of gravity on the stone. So Ws is pulling down on the stone, and Ws is pulling up on the earth.
 
The force of gravity from the Earth on the stone is equal to and opposite of the force of gravity from the stone on the Earth, in accordance with the third law.

The force exerted on the stone by the water is a different force with a different force pair. See the above responses.
 
To clarify what the others have said, the force exerted by the water on the stone is a viscous drag force. This same drag force is also exerted by the stone on the water causing the water to be displaced as the stone accelerates due to the gravitational force.

Additionally, the gravitational force exerted by the Earth on the stone is equal to the gravitational force exerted by the stone on the earth. Therefore, Newton's 3rd law in all cases is not violated.
 
You could have the same situation in air if that helps you intuitively. Air does exert a force just not very big.
 
Ramanujan143 said:
Air does exert a force just not very big.
Actually in all cases the force varies with velocity and can grow exponentially large enough to balance the gravitational force when the terminal velocity is reached.

And of course there is also a buoyant force acting on the stone.
 

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