# Why normal force always prependicular?

1. Dec 1, 2009

### supremesaiyan

why normal force always perpendicular?
why doesn't be in the same direction of the affecting force?
like a mass on a sliding wall why doesn't be opposite the direction of the weight?

2. Dec 1, 2009

### stewartcs

Normal is a mathematical term that means perpendicular.

The normal reaction that you seem to be referring to is called normal since it acts perpendicular to the bodies weight (e.g. the normal force on a book by the table it is sitting on is perpendicular to the book). Hence, it is in the opposite direction of the applied force (the objects weight in this case).

CS

3. Dec 1, 2009

### arildno

Oops, stewartcs:

A normal force acts perpendicularly with respect to a given surface onto some object in contact with that surface.

4. Dec 1, 2009

### stewartcs

Isn't that what I said?

CS

5. Dec 1, 2009

### arildno

You say:
1.
This doesn't have much meaning; normal since it is PERPENDICULAR to the body's weight??

It is normal to the surface if the wall is vertical, and INCIDENTALLY normal to the body's weight.

2.
How can anything be perpendicular to a BOOK??

3.
Now, you contradict yourself with respect to the example in 1, and not the least in the OP's question:

6. Dec 1, 2009

### stewartcs

Oh...LOL...I see what you mean...that did come out wrong initially...

My example is of a book lying flat on a table. The book has some weight (mg) to it. There is a normal force from the table (opposite in direction from mg) applied to the surface of the book which is lying horizontally on the table. Thus making the normal force perpendicular to the surface of the book.

CS

7. Dec 1, 2009

### rcgldr

As mentioned in math and physics, normal means perpendicular to something, either a surface or direction of travel.

Forces aren't always "normal", but generally forces can be separated in to components tangental and perpendicular to a surface.

In the case of an object moving in a specific direction, then the perpendicular component to a line is a plane, so you need additional criteria to separate the components of force on that plane. In some cases, like an electron moving in a magnetic, the electron has a direction, the field has a direction, and the normal force has the remaining direction.