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

In summary, 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. This does not defy Newton's third law as the normal force and weight are not third law pairs. The third law pair to weight is the object exerting an equal and opposite upward gravitational force on the earth. When drawing a free body diagram, it is important to introduce a coordinate system and classify forces into two components. This does not affect Newton's third law. The third law pairs for weight are the normal forces on the object by the plane and the normal force acting on
  • #1
sgstudent
739
3
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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  • #2
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.
 
  • #3
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.
 
  • #4
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!
 
  • #5
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.
 
  • #6
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.
 
  • #7
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!
 

1. What is an inclined plane?

An inclined plane is a flat surface that is tilted at an angle, allowing objects to move up or down it with less force than if they were lifted straight up.

2. How does the angle of the inclined plane affect the object's motion?

The steeper the angle of the inclined plane, the greater the force required to move an object up or down it. A shallower angle will require less force.

3. What is the relationship between the weight of the object and the force needed to move it on an inclined plane?

The weight of the object is directly proportional to the force needed to move it on an inclined plane. As the weight of the object increases, so does the force needed to move it.

4. How does friction affect an object on an inclined plane?

Friction acts against the motion of the object, making it harder to move up or down the inclined plane. The steeper the angle, the more friction there will be.

5. Can an object stay at rest on an inclined plane?

Yes, an object can stay at rest on an inclined plane if the force of gravity acting on the object is balanced by an equal and opposite force. This is known as equilibrium.

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