What are some examples of net forces?

2 examples are an accelerating car and a pulley system where one of the loads are accelerating.

In the first example:

As you can see, there are 4 forces acting on the car.

If there is a total net force/resultant force of 0 (##F_(R) = 0##) both vertically and horizontally, the car will either be at rest or moving at a constant velocity in the direction of the engine thrust.

##F_e-F_{f} = 0## ##F_r-F_{g} = 0##

If there is a total net force/resultant force which is greater than zero in the direction of the engine thrust (##F_(R)>0 ##) horizontally, the car will be accelerating at a constant rate in the direction of the engine thrust as there is a force that makes it go faster.

##F_e-F_{f} > 0## ##F_r-F_{g} = 0##

There is net force only when there is an which is explained by ,

##F=ma## where ##F## is the net force, ##m## is the mass and ##a## is the acceleration.

When ##a## is 0, ##F## will be 0 regardless of the value of ##m##

Similarly, in the second example:

The force from the weight of ##m_2## will be larger than that from the weight of ##m_1##, therefore there will be a net force acting in the downwards direction, pulling the whole system down.

Let's do a simple calculation of the acceleration of ##m_2## using Newton's Second Law:

Assuming ##m_2## to have a mass of 5 kg and ##m_1## to have a mass of 3 kg:

Their weights would be 50N and 30N respectively.

The 50N weight will overcome the 30N weight, leaving a net force of 20N in the direction of the 50N weight.

##F_R## ##=50N-30N## ##=20N##

This net force of 20N is pulling the whole system which consists of both weights, which has a mass of 8kg altogether.

Applying Newton's Second Law,

##F=ma## ##20=(5+3)a## ##20=8a## ##a=2.5m s^-2##

In summary, there is a net force in the direction of acceleration when an object is accelerating.