If you're still reading this after looking at the title, either you have an upcoming Physics exam or you want to be transported back to the days of secondary school, the land of chalk dust, ugly blazers and Physics lessons with that slightly crazy teacher on a Tuesday afternoon. Understandably, on another dull, dreary day you have been staring out of the window, maybe dreaming of having a kick about in the street with your mates. Cast your mind back and try to recall those three laws of motion etched in the minds of every student of Physics; Newton's Laws of Motion.
Specifically, try to remember Newton's Third Law. Naturally, unless you happen to be studying Physics, you have erased it from your mind, so here it is, in it's full form; For Every Reaction, There Is An Equal And Opposite Reaction.
This might not appear to say much, but what it really means is that, every time something happens where a force is acting on an object, there are in fact two forces acting. This second force is the exact opposite of the first force, it is in the opposite direction and exactly equal in size. In simpler words, Newton's Third Law is: For Every Push, There Is An Equally Large Pull. All forces come in pairs.
Hang on a second though, if every force comes with an equal force in the opposite direction, how can things move at all? Shouldn't Newton's Third Law mean that, whenever a force is applied to an object to start it moving, an equal force in the opposite direction should stop it moving? Is Newton actually wrong?
In short, no. Think of it like this: a fish swims through water by pushing on the water with its fins. This action pushes the water around the fish backwards. Since every action has an equal and opposite reaction, the fish is pushed forward. Newton's Third Law does not prevent motion, it enables it to happen.
Take another example: firing a gun; the bullet flies forward at incredible speed, whilst the gun kicks backwards; recoil. At first, it may not seem that the forces are equal, but the bullet has a much smaller mass, making it much easier to accelerate to a higher speed than the gun while applying the same force. The key to Newton's Third Law is remembering that the two equal and opposite forces don't necessarily act on the same object.
What about rockets in space though, they have nothing to push against, there is no air in space, so how can rockets fly? Newton's Law still holds true, it applies to the entire Universe, not just one small rock near an almost completely unremarkable star (this is one of the beauties of Physics, and at the same time, one of the greatest mysteries). The rocket does not need to push against anything, it burns up fuel and shoots the exhaust gases out of the back of the rocket. This is a backwards force, so, of course, there must be an equal force in the forward direction, the rocket is moving forward.
Finally, a little bit of a question: If there is Gravity on Earth (which there is), what stops us from falling into the centre of the world, and crucially, how?