Newton’s Third Law

NEWTON’S THIRD LAW

This law is basically about symmetry in nature. Forces will occur in pairs so that when a force is applied to a second body, the force on the initial body will be equal in magnitude and in the opposite direction of the force initially applied. These forces do not cancel each other out because they are acting on different systems. When a swimmer pushes away from a wall and moves, there is movement because the swimmer pushes on a different system from itself (which is the wall). The force that moves the swimmer will be in the opposite direction to the push against the wall.

Rockets move forward by expelling gas in a backwards direction. There will be a large reaction force on the rocket, known as “thrust”. It is caused by the expulsion of gas in a system, such as a plane or rocket. Rockets do not push against the ground or the air behind them and will actually also work better in a complete vacuum (because of a lack of air pressure and air friction). Birds, helicopters, and airplanes also fly by exerting force on the air that is opposite to the direction they want to go.

In the case of pushing an object, there will be the forward force of pushing the object and opposing forces, such as friction and air resistance. Neutral forces, will be the downward force of gravity as this is an equal and opposite force acting on the object by the earth. Friction will be the major force involved and depends on the characteristics of the object and the substance (such as the ground) that is up against the object. Remember that friction only applies if two objects are touching each other in some way.

You should also know that forces between components of a system will cancel each other out because they are within a system and are said to be equal and opposite in direction. In the situation of pushing a cart, for example, if the system includes the person doing the pushing and the cart itself, there will be no net movement of these objects. There are internal forces between the cart and the pusher that do not affect the movement of the pusher and cart together. If the system does not include the pusher, the pusher can exert a certain external force on the cart, thus moving the system (which is just the cart).