Newton’s Third Law, originally published in his seminal work Philosophiæ Naturalis Principia Mathematica (Principles of Natural Philosophy), was the final of three which changed our fundamental understanding of the universe by explaining mathematically how the motion of objects works.
Newton's Third Law or the Law of Reciprocal Actions
For every action there is an equal and opposite reaction.
This law is often expressed colloquially with the phrase "If you push something it will always push back." For example, a train being pushed along a track is "pushed back" by a frictional force acting in the opposite direction but with the same magnitude of force.
How Gravity Fits In
This everyday situation is easy to comprehend, but a more tricky example could be something like a ball falling through the air. The ball is being pulled downwards by the gravitational attraction of the Earth. This law, however, means that the Earth is also being pulled upwards by the gravitational attraction of the ball! It may seem absurd that something as small as a tennis ball can attract something as massive as a planet, but this is in fact the case. The difference is that the amount the Earth is "pulled upwards" by the ball is so tiny in comparison with the amount the ball is "pulled downwards" that the movement of the Earth is not noticeable.
Mass Comparisons
Another good example is to return to the astronaut in space. When he pushes himself away from his spacecraft, why does he move away from the spacecraft rather than the spacecraft moving away from him? The spacecraft's movement is not being opposed by friction, as it would be if it were sitting stationary on the landing pad, so what keeps the spacecraft stationary?
The answer is of course that the spacecraft does not remain stationary; it will push itself away from the astronaut, but because it has a much bigger mass than the astronaut it does not move very far, whereas he moves much further because of his lower mass. Think of two skaters on an ice rink pushing against each other; if they are the same mass and push with equal force, neither of them will move.
Reference
Newton's Laws of Motion, online at NASA's Glenn Research Centre; Beginner's Guides, accessed May 2010.
