Net force 

Whenever an object moves, it means that a net force is applied to the object

Why not just call it force? It is because more than one force can be applied to an object.

When more than one force is applied to an object, it is more precise to say net force or total force.

The unit of force is Newton. Near the earth surface, a mass of 1 kg has a force of about 9.8 N.

Look at the weight below and answer the following questions

1) How many forces are pulling this weight?

2) What is the total force?

3) Which direction will the weight go?

1) Three forces are pulling the weight. Each one is represented with an arrow pointing to the right.

2) The total force is 5N + 1 N + 8N  = 14 N?

3) The weight will go to the right since all the forces are pulling to the right? 

Answer the same questions with the figure below:

1) there are still three forces pulling the weight. This time though, one force is pulling to the left.

2) This time to get the net force, you will need to add all forces pulling to the right. Then, add all forces pulling to the left. Finally, identify where the force is weaker and where the force is stronger.

Do this math: stronger force - weaker force

Forces pulling to the right  = 1 N + 5 N = 6N

The only force pulling to the left is 8 N.

stronger force - weaker force = 8 N - 6N = 2N

3)  Since the stronger force is on the left,  the weight will move to the left. 

What happens when the net force equals zero?

We call this situation equilibrium. For example, the following situation is in equilibrium.

We put the weight on the table (shown in brown) and some forces are applied.

A total of 6 forces are applied. Let's break it down.

The 3 forces on the right cancel with the force on the left since

8 N - 8 N  = 0

The object is pushing down on the table with a force of  20 N.

However, the table is pushing up also with a force of 20 N.

We call the force the table is applying support force or normal force.

Just because the table is not a human does not mean it cannot exert some force.

Put your hands on a table and push down. Why exactly is stopping your hand from going down?

It is the support force that the table is exerting on your hands.  

Thanks for reading! 

Back to definition of inertia or Newton's first law

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