# Object on an inclined plane

1. Feb 19, 2012

### sgstudent

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!

2. Feb 19, 2012

### skazis

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. Feb 19, 2012

### Staff: Mentor

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. Feb 19, 2012

### sgstudent

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. Feb 19, 2012

### Staff: Mentor

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. Feb 19, 2012

### skazis

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. Feb 19, 2012

### sgstudent

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!