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# What is the normal force? watch

1. So on a macro level or a physics frame of reference, the normal force (Based on my knowledge) exists to counter the force of gravity. What i am struggling to understand is....

How does the normal force exist? (i.e. what causes it to exist?)

FEEL FREE TO ALSO EXPLAIN IT ON A MICRO LEVEL (MOLECULAR LEVEL) BUT I WOULD ALSO APPRECIATE IF YOU EXPLAINED ITO ON A MACRO LEVEL

Does the normal force always equal the forces applied to it?

Can the normal force be overcome. (i.e. does the normal force have a threshold?)

E.G. SUPPOSE A BRICK IS PLACED ON A TABLE, MORE & MORE FORCE IS APPLIED TO THIS SAME BRICK, WHICH IN TURN EXERTS A FORCE ON THE TABLE, CAUSING THE BRICK TO BREAK THROUGH THE SURFACE OF THE TABLE. DOES THIS MEAN THE NORMAL FORCE HAS BEEN OVERCOME?
2. Hi

The normal force is the reaction on an object that lies perpendicular to the plane that the object is on. For example: Consider a book lying on a table. The weight of the book acts vertically downwards. The normal force (reaction) acts vertically upwards.

The normal force exists because if a force is applied to an object with no opposing force, it will accelerate (F = ma). For a book lying on a table, there are only two forces to consider: its weight acting down and the normal reaction. As the book is stationary, there cannot be any resultant force and so they cancel themselves out.

If a vertical force was applied to the book (with the intention of lifting it off the table), then if the vertical force is less than the weight of the book, then the normal reaction is the difference between the two. If the vertical force is equal to the weight of the book, the book is on the point of moving and the normal reaction will be zero.

Regarding your analogy with the brick, there would be other things to consider, e.g. the maximum pressure that could be applied to the table before it breaks. Until that occurs, the normal reaction would be equal to the sum of the vertical component of any force and the weight of the brick.
3. The normal force acts in the direction that is perpendicular to the direction of possible motion of an object.
4. (Original post by B_9710)
The normal force acts in the direction that is perpendicular to the direction of possible motion of an object.
I think you mean opposite rather than perpendicular (otherwise reaction force due to gravity would be push you left or right)
5. (Original post by samb1234)
I think you mean opposite rather than perpendicular (otherwise reaction force due to gravity would be push you left or right)
No because possible motion on the ground is horizontal left or right. So the normal force is perpendicular to that - vertically upwards.
6. (Original post by B_9710)
No because possible motion on the ground is horizontal left or right. So the normal force is perpendicular to that - vertically upwards.
I don't think that is a great way to define it - I inferred, and as may others, that you were talking about the possible direction of motion due to force itself. Also there are certain scenarios where the possible motion isn't horizontal - e.g. you might be drilling a well
7. (Original post by samb1234)
I don't think that is a great way to define it - I inferred, and as may others, that you were talking about the possible direction of motion due to force itself. Also there are certain scenarios where the possible motion isn't horizontal - e.g. you might be drilling a well
We are talking about very simple motion, of a particle or rod. There are some questions where people think that the normal force is acting perpendicular to the surface that the object is on, but you can end up getting it wrong if you think about it like that.

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Updated: March 15, 2016
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