Does the Normal Force Defy Newton's Third Law on an Inclined Plane?

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Discussion Overview

The discussion revolves around the interpretation of Newton's third law in the context of an object resting on an inclined plane, specifically examining the relationship between the normal force and gravitational force. The scope includes conceptual clarifications and technical explanations related to forces on inclined planes.

Discussion Character

  • Conceptual clarification
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether the normal force defies Newton's third law since it is not directly opposite the weight of the object on an inclined plane.
  • Another participant explains that the gravitational force can be split into components, with one component opposing the normal force and the other potentially causing acceleration if friction is overcome.
  • Some participants clarify that the normal force and weight are not third law pairs, as weight is the force exerted by the Earth on the object, and the corresponding third law pair is the force exerted by the object on the Earth.
  • There is a discussion about drawing free body diagrams and whether forces should be split into components, with some participants indicating that this is a separate consideration from Newton's third law.
  • Participants express understanding and appreciation for the explanations provided, indicating a collaborative atmosphere in clarifying these concepts.

Areas of Agreement / Disagreement

Participants generally agree on the interpretation that the normal force and weight are not third law pairs. However, there is some debate regarding the implications of force components and their relation to Newton's third law, indicating that the discussion remains somewhat unresolved.

Contextual Notes

There are limitations in the discussion regarding the assumptions made about the definitions of forces and the conditions under which they are analyzed, particularly concerning the role of friction and the orientation of the coordinate system.

sgstudent
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Newton's third law states that with every force, there is an equal and opposite reaction and these forces will act on mutually opposite bodies.

However, when an object is resting on an inclined planed, the normal force is not directly opposite of the weight so doesn't this defy Newton's third law in a way?

Thanks for the help!
 
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Force caused by gravitation will be split into two components depending of your inclination. One will be opposite to the normal force, but other will make your object accelerate if friction force is surpassed.
 
sgstudent said:
However, when an object is resting on an inclined planed, the normal force is not directly opposite of the weight so doesn't this defy Newton's third law in a way?
No. The normal force and the weight of an object are not third law pairs. Weight is the Earth exerting a downward gravitational force on an object. The third law pair to weight is thus the object exerting an equal and opposite upward gravitational force on the earth.
 
skazis said:
Force caused by gravitation will be split into two components depending of your inclination. One will be opposite to the normal force, but other will make your object accelerate if friction force is surpassed.

Oh thanks this makes a lot of sense :) so one of the component is to overcome the weight then the other component is to overcome the friction is there is any movement at all. But when u draw the free body diagram I don't split them up right?

Thanks so much u rock!
 
Careful here. While it's usually a good idea to split forces into components parallel and perpendicular to the incline, this has nothing to do with Newton's 3rd law.
 
sgstudent said:
Oh thanks this makes a lot of sense :) so one of the component is to overcome the weight then the other component is to overcome the friction is there is any movement at all. But when u draw the free body diagram I don't split them up right?

Thanks so much u rock!

As I understand, usually a coordinate system (CS) must be introduced. Then forces must be classified into two. First half will be on the x axis, the other - y. In the case of gravitation - you draw it, as well as its components in this CS.
 
Doc Al said:
No. The normal force and the weight of an object are not third law pairs. Weight is the Earth exerting a downward gravitational force on an object. The third law pair to weight is thus the object exerting an equal and opposite upward gravitational force on the earth.

Oh yea! Weight acts on itself so the third law pairs are the normal foxes on the object by plan and normal force acting on plane by object! Got it!

Thanks so much! You rock!
 

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