# Opposing forces and Newton's third law

1. Nov 13, 2014

### Scheuerf

According to Newton's third law, every force has an equal and opposite reaction force. Which force is equal and opposite to both friction and tension?

2. Nov 13, 2014

### Staff: Mentor

Newton's 3rd law is telling us that every force is its own pair. So the force opposite friction is friction and the force opposite tension is tension. For example, if you slide a box across the floor, the same friction force acts in one direction on the box and in the opposite direction on the floor.

3. Nov 13, 2014

### A.T.

That is a confusing way to put it. I would rather say:

Each force of a certain type has an equal but opposite counterpart of the same type.

4. Nov 13, 2014

### Godparicle

The question is not clear.

The below words from Wiki will help you understand Newton's law better:
"Whatever draws or presses another is as much drawn or pressed by that other. If you press a stone with your finger, the finger is also pressed by the stone."

I hope you can form the better question now. In the way, you might even get an answer. Good Luck.

5. Nov 14, 2014

### Bendelson

Both forces in a force pair described in Newton's third law exist but are not both acting on the same object, otherwise objects would all be in equilibrium. The force of tension on an object from a rope has a force of an equal magnitude on the other end of the rope.
A good example of force pairs is the fact that the force of gravity applied on you by the Earth is EQUAL to the gravitational force you apply to the Earth all though opposite in direction. These 2 forces- you're gravitational pull and the Earth's gravitational pull- are the reaction pairs.
As you mentioned friction, when you rub your hands together and were to draw an FBD for each hand, you would find that a friction force is acting on each hand but in the opposite direction of the friction on the other hand.
Like matter and energy, force cannot be created or destroyed. In every situation that a force is applied, another force of equal magnitude that is opposite in direction is applied to another object.

6. Nov 14, 2014

### jbriggs444

The fact that forces come in third-law pairs means that the a system subject to no net external forces will also have no net internal force. That supports the idea of conservation of momentum. You might say that momentum can never be created or destroyed, just moved from one object to another.

Forces are as easy to create or destroy as candle flames. If you stop leaning on the wall, that force is gone!

7. Nov 15, 2014

### Orodruin

Staff Emeritus
This is a bit misleading. The sum of all internal forces are always zero.

8. Nov 15, 2014

### Bendelson

*the sum of all the forces in a full system are zero.
If the sum of forces on every object was zero, nothing would move because everything would be in equilibrium

9. Nov 15, 2014

### Orodruin

Staff Emeritus
This does not in any way contradict what I said. The sum of all internal forces being zero obviuosly does not mean yhere are no internal forces.