# Why Pressure is a scalar quantity?

1. Jan 3, 2007

Hi,

1. As we know that pressure is force per unit area but why it is said that pressure is a scalar quantity because force is used and force is a vector quantity.

2. Whether stress is also a scalar quantity?

3. What is basic difference between pressure and stress?

Regards,

2. Jan 3, 2007

### Celt67

Very interesting question.

Pressure as a scalar quantity is debatable. I think that it is a scalar quantity because it is per unit area. There is no direction involved as such.

I think pressure deals with liquids and stress is for solids. I could be wrong.

3. Jan 3, 2007

### Staff: Mentor

Pressure does have a direction. What is that direction with respect to a surface if the air is not moving, for example. What about when you have air going over an airfoil? What are the components of the pressure then?

4. Jan 3, 2007

### Celt67

I'm way too tired to get into a discussion like this or make any useful contribution! This is something physics professors like to chat about, or so I've heard!

5. Jan 3, 2007

### f(x)

Well, pressure is a scalar for any point inside a gas=>assume a point inside an ideal gas chamber. Pressure from all sides shall be equal (for steady state)Thus you cannot predict the direction.
However on surfaces, solid liquid and gases, pressure has a definite direction perpendicular to the surface.

However, could some1 plz elaborate on pressure inside a liquid.....does that also not have a direction? (i mean, pressure varies with depth, so at one height, will all the pressure act at a particular point or will the direction be indeterminate ?)

6. Jan 3, 2007

### Staff: Mentor

Boy, that NASA article had me worried there for a minute, with them saying pressure is definitely a scalar quantity. But then I saw what I was referring to, which is what they call the "pressure force", which definitely is a vector:

So I learned from this thread and that link to be more precise in the way I talk about pressure and "pressure force". Thanks for the link, f(x).

7. Jan 3, 2007