Newton’s 3rd Law of Motion is the most famous of the three, but also the most misunderstood: For every action, there is an equal and opposite reaction. To understand what it means, place your hand against a wall, and push. You are now applying a force to the wall through your hand, but your hand does not move. Why? Because the wall is exerting a reactionary force against the hand, exactly equal to your force, but in the opposite direction. These two forces cancel out, and so no movement happens (although they are still there – you can feel them pressing against your hand). Now try another experiment: stand on a skateboard, and throw a basketball forward as hard as you can. Assuming you keep your balance and stay on the skateboard, you will find that you roll backwards as the ball flies forwards. You have exerted a force on the ball, which then exerted a reactionary force back on to you. Your push on the ball causes it to move in the direction of the force (forwards), and the reaction force on you causes you to move in the opposite direction. The force on the ball was the same magnitude as the force on you, but you are heavier than the object and so you moved more slowly (thanks to Newton’s Second Law).