1. Newton's third law states that when two objects interact, they exert forces on each other that are equal in size and opposite in direction.
2. The two forces form an interaction pair, often called a third law pair.
3. Every third law pair has the same set of features.
   1. The forces are equal in size.
   2. The forces act in opposite directions along the same line.
   3. The forces act on different objects.
   4. The forces are the same type of force, such as two gravitational forces or two normal contact forces.
   5. The forces start and stop at the same time.
4. The law applies to every interaction, whether the objects are touching or not, and whether they are moving or still.
5. A short way to state a pair is "if A exerts a force on B, then B exerts an equal and opposite force on A".

Third law pairs and balanced forces

1. Third law pairs never cancel each other out, because the two forces act on different objects.
2. Forces can only cancel when they act on the same object, and then they are balanced forces.
3. A mug resting on a desk shows the difference clearly.
   1. The weight of the mug (the Earth pulling the mug down) and the normal contact force (the desk pushing the mug up) both act on the mug, so they are a pair of balanced forces, not a third law pair.
   2. The third law partner of the mug's weight is the mug pulling the Earth upwards.
   3. The third law partner of the desk pushing the mug up is the mug pushing the desk down.
4. Two forces that are equal and opposite are therefore not automatically a third law pair, so you must check which object each force acts on.

Situations in equilibrium

1. In equilibrium situations, the object you are studying has balanced forces on it, but third law pairs are still present between it and every object it touches or attracts.
2. When you lean on a wall without moving, your hand pushes the wall and the wall pushes your hand back with an equal force, which you feel in your palm.
3. When a picture hangs on a hook, the picture pulls down on the hook and the hook pulls up on the picture with an equal force.
4. When two teams in a tug of war are not moving, each team pulls the rope and the rope pulls back on each team with an equal force.

Situations that are not in equilibrium

1. Third law pairs also act when objects are speeding up, slowing down or changing direction.
2. Both objects feel the same size of force, but the effects of the forces can be very different because the objects can have very different masses.
3. When you walk, your foot pushes backwards on the ground and the ground pushes forwards on your foot with an equal force, which accelerates you forwards.
4. When a swimmer pushes water backwards with her arms, the water pushes her forwards with an equal force.
5. When a rocket fires its engines, it pushes hot exhaust gases downwards and the gases push the rocket upwards with an equal force.
6. When a gun is fired, it pushes the bullet forwards and the bullet pushes the gun backwards with an equal force, so the gun recoils.
   1. The bullet has a tiny mass, so it gets a huge acceleration.
   2. The gun has a much larger mass, so it gets a much smaller acceleration.
7. When a car hits a fly, the force on the fly is exactly equal to the force on the car, but the fly is squashed because its mass is so small.