Violation of Newton's third law

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SUMMARY

The discussion clarifies that Newton's third law of motion is not violated when a stone sinks in water. The gravitational force (Ws) acting on the stone is greater than the upward force exerted by the water (Fw), resulting in the stone's downward acceleration. The forces between the stone and the water are distinct, with the water exerting a viscous drag force on the stone, which is equal and opposite to the force exerted by the stone on the water. Additionally, the gravitational forces between the Earth and the stone adhere to Newton's third law, confirming that all forces involved are balanced in accordance with the law.

PREREQUISITES
  • Understanding of Newton's laws of motion, specifically the third law.
  • Basic knowledge of gravitational forces and buoyancy.
  • Familiarity with concepts of viscous drag forces.
  • Awareness of terminal velocity and its implications in fluid dynamics.
NEXT STEPS
  • Study the principles of buoyancy and Archimedes' principle.
  • Learn about viscous drag forces and their calculations in fluid dynamics.
  • Explore terminal velocity and its factors in different mediums.
  • Investigate real-world applications of Newton's laws in fluid mechanics.
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Physics students, educators, and anyone interested in understanding the principles of motion and forces in fluid dynamics.

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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