Applied, Contact and Normal Forces

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Description

This is a basic physics tutorial that is targeted at GCSE (grade 9 and grade 10) standard reviewing applied forces, contact forces and normal forces. The video for this written tutorial is on its way.



What is a contact force?

When a person stands on the earth, they are experiencing a downward pulling force, pulling them towards the centre of the earth. But the ground doesn't open up and swallow the person. Similarly, when a block rests on a table, the block doesn't fall through the table.


Why does this happen?

This happens because there are contact forces exerted between both objects i.e. not just the block on the table, but also the table on the block. These paired forces are in compliance with Newton's Third Law of Motion - that for every action there is a reaction.

They are also in compliance with Newton's First Law of Motion. How so? Because they are equal in magnitude, but opposite in direction, they balance out to result in the uniform motion of staying at a resting state i.e. the block doesn't move, and neither does the table.

What are applied forces?

The contact forces described above can also be referred to as applied forces because they only exist if one object is applying a force on the other. i.e. not when they are simply touching at rest (like the two objects A and B below).

Objects A and B are simply touching one another at rest; they are not exerting any forces on each other. Hence, there are no contact or applied forces being exerted on either of them by the other. They will however, both be applying downward forces on the shelf they are sitting on; and the shelf will be exerting forces of the same magnitude but opposite direction on each of them.


Balancing forces between a block on a table

In the case of the block on the table, the gravitational pull exerted on the block by the earth results in it being drawn towards the centre of the earth. This results in a contact force with the table, that is equal to the weight of the block.

There is also a contact force exerted by the table on the block, equal in magnitude, but opposite in direction. Interestingly, these forces can also be referred to as normal forces.

Hover your mouse over the image below to see the paired balanced contact forces


What is a normal force?

A normal force is any applied force that occurs at a normal angle (i.e. at 90 degrees) to the surface of touching objects (also known as the contact surface). Both of the contact forces above can be described as normal forces.