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Why Pressure is a scalar quantity? 
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#1
Jan307, 10:14 AM

P: 12

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, Muhammad Rizwan Khalil 


#2
Jan307, 10:24 AM

P: 10

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'm not sure about stress. I think pressure deals with liquids and stress is for solids. I could be wrong. 


#3
Jan307, 10:27 AM

Mentor
P: 41,106

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
Jan307, 10:29 AM

P: 10

Why Pressure is a scalar quantity?
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
Jan307, 10:35 AM

P: 183

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. EDITI found an interesting link which might be helpful 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
Jan307, 10:51 AM

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P: 41,106

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:



#7
Jan307, 10:54 AM

P: 12

How its does have a direction in solids but not in gases?



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