# Based on Newton's third law

1. May 13, 2015

### hussianm

Do weight and normal contact force always act in equal and opposite directions?

2. May 13, 2015

### Staff: Mentor

Welcome to the PF.

What do you think, and why?

3. May 13, 2015

### hussianm

Well I saw this question: Newton's third law tells us that... One of the options was that weight and normal contact force are always equal and opposite and the correct answer was forces always arise in pairs. The other option were wrong :p
The correct option makes perfect sense but I wanted to know why the other statement is not correct.. Is it because there is not necessarily a contact force when there is weight acting on a body?

4. May 13, 2015

### HallsofIvy

If an object is sitting on a horizontal surface, like a table or a floor, then its weight is a force downward and normal force upward. Since the object is not moving, those two must be the same (and in opposite directions).

If on the other hand, an object is sitting on a slope, on a slanting surface, gravity is downward, while the normal force is NOT upward. It is "normal" to the surface. In order that the object not move down the slope, there must be a friction force, tangent to the slope. The sum of those two vectors must be equal (and opposite) to the weight force.

5. May 13, 2015

### hussianm

oh! i see. i understand now. thank you for your help

6. May 13, 2015

### PhanthomJay

HallsofIvy's response was an excellent example of Newton's first law. Newton's third law tells you that if the object exerts a normal force N on the table, then the table must exert an equal normal force N on the object, in the opposite direction. This is the force pair of the normal force. It is important not to confuse his first law with his third.