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## Physics library

### Course: Physics library>Unit 9

Lesson 1: Density and Pressure

# Pressure at a depth in a fluid

Sal derives the formula to determine the pressure at a specific depth in a fluid. Created by Sal Khan.

## Want to join the conversation?

• when he says force downward is equal to force upward, he makes force downward equal to the weight of water but what about the weight of the foil? it is also acting downward.
• For the purpose of this example, you will have to imagine that the foil is massless. But you are correct, in a real world simulation, there is a gravitational force acting on the foil as well.
• Since: F = P(rho) * V * g and P = mass/V, (which would mean that F = Mass/V * V * g), couldn't the V in Mass/V and the other term 'V' cancel out to leave us with F = Mass * Gravity?
• Force = (Mass *acceleration due to gravity) not (Mass*Gravity) because here Force = Gravity and g = acceleration due to gravity
• What is the force up caused by?
• the force up is called Buoyant force is an upward force exerted by a fluid that opposes the weight of an immersed object.
Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.
— Archimedes
In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus a column of fluid, or an object submerged in the fluid, experiences greater pressure at the bottom of the column than at the top. This difference in pressure results in a net force that tends to accelerate an object upwards.
• What is 1 Newton and 1 Pascal?
• 1 newton is the amount of force needed to accelerate 1kg at a rate of 1 meter per second squared. A pascal is a measure of pressure and is equivalent to 1 newton per meter squared.
• If there is vaccum above, wouldn't the liquid go there?
• good question

the liquid would tend to evaporate into the vacuum but gravity would stop it from all moving up
• What are the laws of flotation?
• Something will float if it less dense the fluid that it is in.
The buoyancy force experienced by the object is equal to the weight of the fluid that it displaces. So a brick will sink in water as it will displace a volume of water that is the same as its volume but as it more dense the upward buoyancy force will be less than the brick's weight.
• Is the height measured from the surface of the fluid or from its bottom?
• In this example, h is the height measured from the surface.
• How to convert newton to pascal?
• 1 Pa = 1 N/m²
• Downward force: caused by weight of water above it
Upward force: caused by what?