# Force and Net Force

In theory books, the concept of force is expressed as a push or pull upon an object causing an object's state of motion to change. However, this definition does not explain the concept of force entirely.

We might more correctly say that a force is a push or a pull upon an object resulting from its interaction with another object.

Whenever there is an interaction between two objects, there is a force upon each of the objects. When the interaction ceases, the two objects no longer experience the force.

Forces are usually represented with arrows. The arrow shows: the size of the force (the longer the arrow, the bigger the force) and the direction in which the force acts.

Figure 1 highlights another fundamental concept: the net force. The net force on an object is the combined effect (the sum) of all the pushing and pulling forces actually acting on the object. If the forces pushing or pulling on an object are not balanced (a net force acts) then the object will accelerate in the direction of the net force.

Figure 1: the forces pushing or pulling on the two shopping carts are not balanced. In fact the cart is first experiencing a net force of 60 N + 30 N = 90 N to the right, and then a net force of 30 N - 10 N = 20 N to the left. Notice that the carts will always accelerate in the direction of the net force.