Explain Newtons Third Law - Linear Motion

Stating that a force is just a push or a pull is a bit of a simplification.  If we exert a push or a pull on an object then the object must push or pull back in order for the interaction to take place. 

 In order to push on the wheelbarrow, the wheelbarrow must push back on the man.  The hammer exerts a force on the nail but the nail exerts an equal force on the hammer in the opposite direction. 

This leads to Newton's Third Law:

Action and Reaction - Whenever one body exerts a force on a second body, the second body exerts an equal and opposite force on the first. 

Sometimes it is more quickly worded:

For every action there is an equal and opposite reaction.

This means that forces come in pairs.  We can call them action-reaction pairs.  It is kind of like a poem: When object 1 acts on object 2, object 2 must also act on object 1.                  

Example 1 - As you walk across the floor, your feet exert a force on the floor (action) and the floor exerts an equal and opposite force on your foot pushing you forward (reaction).

 Example 2 - This law of motion drives rocket propulsion.

As hot gases are pushed out the bottom of a rocket (action), the gases push upward on the rocket with the same force, (reaction) accelerating the rocket so that it can escape the Earth's gravitational field.  

Example 3 - If two people of roughly equal mass stand next to each other on an ice skating rink, when one person pushes the other both skaters will move. The skater who pushes exerts a force to the right on the second skater but the second skater doesn't have to actively push the other. By Newton's third law of motion, the first skater will experience an equal and opposite force (to the left) to the push he gives the second skater. Both skaters move away from each other.   

Example 4 - Suppose we have two bodies with very different masses like a Volkswagen Bug and a flying insect. If the insect hits the windshield of the Bug, the windshield will push forward on the bug with the same force as the bug pushes backward on the windshield of the car. But they do not undergo the same change in speed. The car's velocity doesn't change much (not much acceleration) because it is very massive, whereas the insect's mass is very small, so it experiences very LARGE speed change (lots of acceleration) - SQUASH!