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It is weird to ask but i want to know if there is any case where the action force can be more than the reaction force.

This question came into my mind when I thought that if a person sits on chair which is weak it breaks. So is it that, that the reaction force applied by chair was less than the action force or the weight of person??

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  • $\begingroup$ Sure. On top of a roller coaster ride when the rider feels weightless, the reaction force is less than the weight (action). $\endgroup$ Commented May 24, 2018 at 13:48

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The event of breaking of the chair is purely attributed to the material characteristics, and does not violate Newton's Third Law. The material of the chair may have been pre-stressed or may have suffered fatigue loading conditions. Whatever the situation might be, AS LONG AS THE CHAIR IS IN CONTACT WITH THE PERSON, it exerts the same reaction force as exerted by the person. It is just that the material of the chair cannot withstand the applied load. Thus, the chair breaks, breaking off contact between the chair and the person, and essentially releasing the stress.

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  • $\begingroup$ So what about forces like frictional force and restoring force of spring which are limited or fixed at fixed distances respectively Are they not reaction forces? $\endgroup$
    – Murar
    Commented May 24, 2018 at 9:36
  • $\begingroup$ Frictional force is a dissipative force which occurs only when the bodies are in relative motion. It is not a Reaction Force, rather a System Response Force. As for the Spring Forces, if a spring is compressed upto it's maximum compressible length, the internal structure of the spring would resist any further deformation and hence, Action = Reaction (Applied force = Spring force). In case of spring subjected to tension, the maximum elongated length again depends on material. Upto the maximum stretch, Newton's Law holds good. Once stretched beyond that, the spring breaks, thus releasing contact $\endgroup$ Commented May 24, 2018 at 9:41
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Essentially there are three forces acting on the chair which combined together compress the material of the chair. If the compressive stress is too large the material deforms catastrophically and breaks.

The three forces and their Newton’s third law (equal magnitude and opposite direction) pair (+) are:

  • The downward force on the chair due to the gravitational attraction of the Earth + the upward force on the Earth due to the gravitational attraction of the chair.
  • The downward force on the chair due to the person sitting on the chair + the upward force on the person sitting on the chair due to the chair.
  • The upward force on the chair due to ground + the downward force on the ground due to the chair.

When the chair breaks it starts to move down and force that the chair exerts on the person sitting on it is reduced.
You know this to be true because then the person who was sitting still on the chair starts to move downwards.
However at the same time the force that the person sitting on the chair exerts on the chair is also reduced by exactly the same amount so Newton’s third law still holds true.

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  • $\begingroup$ So a restoring force is a reaction force $\endgroup$
    – Murar
    Commented May 24, 2018 at 14:01
  • $\begingroup$ Newton third law parts act on different objects and are of the same type eg both contact forces, both gravitational attractive forces, both electrostatics forces etc. $\endgroup$
    – Farcher
    Commented May 24, 2018 at 15:27
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can there be any case where action force can be more than reaction force

Newton’s 3rd law says that “for every action there is an equal and opposite reaction”. So, no, as long as Newton’s laws apply this explicitly cannot happen.

As this question came into my mind when i thought that if a person sits on chair which is weak it breaks so is it that, that the reaction force applied by chair was less then the action force or the weight of person??

Here you are mis-analyzing the pairs of forces. The weight is a gravitational force between the earth and the person, it does not involve the chair at all.

The interaction between the chair and the person is a contact force. By Newton’s third law the upwards contact force from the chair on the butt is equal and opposite the downwards contact force from the butt on the chair.

Because the chair is weak that upwards contact force on the person is less than the downwards force of the weight, and so by Newton’s 2nd law the person accelerates downwards.

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