# Newton's third law - normal force

• Lori
In summary, the problem statement is asking for the object that the contact force from the floor is acting on. The correct answer is C) ONLY upwards on chair. The word "significant" in front of force is redundant and does not provide useful information.
Lori

## Homework Statement

A book is placed on a chair which is standing on the floor. An iPad is placed on the book. The floor exerts a significant force:

A) only on the book
B) Upwards on chair and downwards on book
C) ONLY upwards on chair
D) upwards on chair book and iPad
E) downwards on upad,book and chair
[/B]

## Homework Equations

Third law, action reaction pair

## The Attempt at a Solution

I was think it would be D) because the normal force has to be able to support the total weight of the chair, iPad, and book. But, I also think it could be C) because the normal force only acts on one object which can only be the chair.

Which is right and why?[/B]

Which objects are in contact with the floor and can therefore exert a contact force on it?

Hey Lori! :)

Extending a little bit on @Orodruin's response, there is a difference between the magnitude of a force and the object that a force acts on.
Which one is the problem statement asking for?

I like Serena said:
Hey Lori! :)

Extending a little bit on @Orodruin's response, there is a difference between the magnitude of a force and the object that a force acts on.
Which one is the problem statement asking for?
I think i understand now! So, the problem isn't asking for the magnitude of the force that is a result of the three objects, but it is asking for the object that this force is acting on.. so all this magnitude of force is acting on the contact object which is only on the chair. Is the key word significant? (if it was asking this way: what magnitude is the floor applying on the chair, it would be the sum of the objects?)

So, in the end, the normal force is always acting on the object that it's in contact with? But it doesnti necessarily equal to the weight of the object?

Yep. All correct!
The word 'significant' in front of force is redundant though. It doesn't tell us anything useful, and seems to only distract us.

I like Serena said:
The word 'significant' in front of force is redundant though. It doesn't tell us anything useful, and seems to only distract us.
I disagree. I believe it is there to avoid smarty pants students saying that all objects affect all other objects gravitationally.

## 1. What is Newton's third law?

Newton's third law states that for every action, there is an equal and opposite reaction. This means that when an object exerts a force on another object, the second object will exert an equal and opposite force back on the first object.

## 2. What is the normal force?

The normal force is the force that a surface exerts on an object that is in contact with it. It is always perpendicular to the surface and acts in the opposite direction of the force applied by the object on the surface.

## 3. How does Newton's third law apply to the normal force?

According to Newton's third law, the normal force is equal in magnitude and opposite in direction to the force applied by an object on a surface. This means that if an object is pushing down on a surface, the surface will exert an equal and opposite force, known as the normal force, on the object.

## 4. Can the normal force be greater than the force applied by the object?

No, according to Newton's third law, the normal force and the force applied by the object are always equal in magnitude. This means that if the force applied by the object is increased, the normal force will also increase to match it.

## 5. How is the normal force affected by the weight of an object?

The normal force is affected by the weight of an object because the weight of an object is the force of gravity acting on it. When an object is placed on a surface, the surface will exert a normal force to support the weight of the object, preventing it from falling through the surface. This means that the normal force will always be equal in magnitude to the weight of the object.

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