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Air and Liquid Pressure

Does air pressure in a capped bottle is different as that of open bottle??

If not and the atmospheric pressure in a closed container is same as that of the surroundings let 1bar at sea level, if i consider a
tube both end open and dip one end in water (like pipette in chemistry lab) and close the other by thumb, water remain
hanged in the tube..  if we say it is because the atmosphere that pushes up on the water in the tube is same as that of remaining air in tube pushing down on the water..won’t the water fall out due to its own weight as the upward and downward pressure is balanced…

Please explain the whole process and compare weight of water with up and down pressure by atmosphere..

Again would liquid ‘ll flow out of a container through a hole in vacuum??


 Asked Shashank Patra

The Air Pressure in an open bottle is equal to the atmospheric pressure. The pressure inside a closed bottle can be different.

In the experiment described, when the tube is partially filled and the upper end is closed, the water tries to fall down creating a lower pressure above it inside the tube. This creates a pressure difference, the outside pressure greater than the pressure inside and the water can fall only upto the level where the weight of water column is balanced by the force due to difference in pressure created.
See one live demonstrations here


  • A guide into hydraulics – Part 1


Buoyant force concept and terminal velocity? My aerodynamics book(John Anderson Aerodynamics) say that whenever a body moves through a fluid, there are only two types of forces acting on the body. 1. Pressure force that acts perpendicular to the plane of the body and the other is the drag force that acts parallel to the objects velocity.


Forces at Work


Also i read an article that buoyant force is related to static fluids not moving fluids. Now, i was working on a concept of terminal velocity of an object moving through air in the Z direction. According to my concept, the air is continuously in motion and hence its a dynamic fluid. So, accordingly, the body falling through air will experience friction drag parallel to the velocity and pressure force perpendicular to the velocity. But as i was going through the derivation of the terminal velocity, they have counted the buoyant force in which is completely confusing me. If there is buoyant force when the body is moving through a moving fluid then when an aircraft is cruising at an altitude, why do they not mention the buoyant force? Rather they say that the vertical force is the pressure force which is also the lift force. I am really confused when to apply buoyancy and the concept of this pressure force which acts perpendicular to the plane of the body Please help me.


Pressure, Thrust, Area and Pascal’s Law

“In Pascal’s law we find that pressure does not increase with area. But when we study about pressure, we learn that pressure is inversely proportional to area. How is this possible? Please explain.”

Fahad Imtiaz asked via Speak Pipe



Pressure is defined as the thrust (the total force acting normal to a surface) per unit area.

Pascal’s law deals with fluid pressure and the statement goes –

“The pressure exerted anywhere in an enclosed incompressible and non-viscous fluid is transmitted equally and undiminished in all directions through out the fluid, provided the effect of gravity is neglected”

Read the statement carefully.

Here we are not changing the definition or meaning of pressure.

the fact to note that, in an enclosed fluid, the pressure is transmitted equally throughout the fluid. Therefore, if we apply some pressure somewhere in the fluid, the same pressure will be felt  at any other place on the enclosed fluid. This gives us an opportunity to multiply the force. Since the pressure is equal everywhere, if we increase the area the force (thrust) is increased.

P = F/A or F = PA

So, pressure remaining constant, greater the area, greater is the force.

Hope you understand the matter now.


a glass jar contains a liquid of density ‘d’ up to aheight ‘h’ at a place where acceleration due to gravity ‘g’. the atmospheric pressure is PA. a) What is the pressure at the free surface of the liquid? b) Write an expression for the total pressure at the bottom of the jar . c) What will be the lateral pressure at this depth on the inner side of the jar?
Asked Jenny

Gravity and something more

Jihin Asked:

Some Things That I Have Always Wondered OF : (i hope it may be answered soon …)
1)is it possible that our human body can withstand pressures twice or more than the atmospheric pressure without a suite or is there any artificial instruments or means in which our heart can be made to pump into a level in which we can attain this ability !!!
2)when more gravity  is experienced , it is said that time travels slowly – is it  relative ??? caz time goes at a constant rate !!! it is only the position or view that makes the observer feel the situations ???
3) Is there any gravity chambers successfully made out … in which the gravity can be changed accordingly to observe diff. phenomena ??? Cant It be possible to do that ????


To be posted soon.

However, I agree that I may not be able to give you a satisfying answer to all your queries, as “Science says the first word of everything and the last word of nothing.” We are learning new things everyday. When we explore more and more, we find that we know less and less and our knowledge is too meagre.

By the time I prepare a well fitting answer to this, visitors are also requested to respond to the question by posting answers and comments.

BP, cardiac arrest and Physics

Ashlin asks:


Devika asks:

Why the blood pressure in humans is greater at the feet than at the brain.

The Physics of a Pressure Cooker

How does a pressure cooker work? What is the Physics behind it?


Pressure Cooker
Physics of Pressure Cooker

A pressure cooker develops a very high pressure when water boils inside. At high pressure the boiling point of water also rises and therefore food gets cooked at a higher temperature. Therefore the food materials are cooked well and faster than at normal temperature and pressure.

A pressure cooker is extremely useful in hilly areas where the pressure is very low and the boiling point is less for water. AT a lower boiling point, cooking is very difficult as proper cooking will not take place. When a pressure cooker is used, it increases the pressure as well as the boiling point and the food materials can be cooked well.

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January 2021

Schrodinger’s Cat in Daily Life

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