You are Here: Home

# What is Pressure? Tweet

Physics and electronics discussion, revision, exam and homework help.

Announcements Posted on
TSR launches Learn Together! - Our new subscription to help improve your learning 16-05-2013
IMPORTANT: You must wait until midnight (morning exams)/4.30AM (afternoon exams) to discuss Edexcel exams and until 1pm/6pm the following day for STEP and IB exams. Please read before posting, including for rules for practical and oral exams. 28-04-2013
1. What is Pressure?
Hi

Why does pressure act in all direction?

Shouldn't it just act downwards due to gravity?

Why is the pressure acting sideways also dependent on the depth?

In general, why is pressure dependent on depth and acts in all directions?

Anyone got the derivation of pressure? (How it was discovered)

Thank you.
2. Re: What is Pressure?
Because the motion of particles is random. Some will be going up, others down/left/right/in/out. I guess it stands true that the surface in the direction of the gravitational force will have a higher pressure than the other surfaces, but it would probably be negligable and doesn't need considering.

From the Unit 5 equation (AQA): pV = 1/3NM(C root mean squared)

And c mean is the average speed of all the particles in all directions. So you can derive the speed/momentum for a particle in relation to a particular direction, but the speed and momentum is equal to every direction therafter.
3. Re: What is Pressure?
Pressure results from particles moving around and hitting surfaces. The ideal gas law links the pressure of a gas to its density and temperature, such that the pressure is increased if the temperature is increased or the density is increased (or both). As mentioned in the previous comment the particles move about randomly, although gravity acts on them it doesn't dominate (just as people can freely walk around) and so pressure acts in all directions.

Pressure is dependent on depth in general because of weight. For example in the ocean, if you are 1 metre down you have the weight of just 1 metres depth of water above you, if you are 500 metres down you have alot more mass above you and so there is a greater force acting, which means a greater pressure (since pressure is the force acting per unit area). If you go up into the atmosphere you will generally find the pressure decreases since there is less mass above you the higher you go. The weight of the atmosphere and oceans is because of gravity so in this way the pressure is a result of gravity but the motion of particles is not always downwards so the pressure is the same in every direction.
Last edited by SDA; 13-06-2012 at 23:04.
4. Re: What is Pressure?
You have just confused me.

If as you dive deeper into the ocean theres more water acted on you that means the pressure is only acting downwards and the side way pressure will not be affected by depth. But it is not true, the pressure increases as you dive deeper regardless of the direction. How does these two things relate thats what I am asking for.

You say it acts in all directions and the gravity is negligeble and then you say gravity affects the magnitude of the pressure... I dont get you?

Thank you
5. Re: What is Pressure?
(Original post by Chonggiy)
You have just confused me.

If as you dive deeper into the ocean theres more water acted on you that means the pressure is only acting downwards and the side way pressure will not be affected by depth. But it is not true, the pressure increases as you dive deeper regardless of the direction. How does these two things relate thats what I am asking for.

You say it acts in all directions and the gravity is negligeble and then you say gravity affects the magnitude of the pressure... I dont get you?

Thank you
The reason why it increases when you dive further is because the force from above increases.

There's more mass (the water above you) being forced towards the centre (the gravitational force; this is where gravity comes in) of the Earth and you're in between so you feel that force on you.
6. Re: What is Pressure?
As mentioned before, the key is to understand that the particles move in all directions, they don't only move downwards. Therefore the particles at any depth are moving in all directions so there is pressure in all directions. The magnitude of the pressure is determined by the force acting on the particles since the particles apply a pressure to balance the force of the weight above it.
7. Re: What is Pressure?
Then shoudn't only the pressure in the direction in-lined with the gravitation force be affected by depth? But not all directions affected by the depth?