WHAT IS NASA PHYSICS?
MODULES
Forces and Motion
Conservation of Momentum & Energy
Temperature and Heat
Fluids
Optics
Electromagnetic Spectrum
Modern Physics
Anticipation Guide 7
Intro to Modern Physics
Blackbody Radiation
The Ultraviolet Catastrophe
The Photoelectric Effect
Bohr's Atom
Spectra
Radioactive Decay
Special Relativity (SR)
Simultaneity
Distance and Time
General Relativity
May the Forces be with You
Modern Physics Notebook
Assessment Problems 7
Useful Things
SITE MAP
Fluids: Liquids and Gases
Pressure in Liquids
Pascal’s Principle and Hydraulic Lifts
Liquids transmit pressure equally in all directions. This means that an increase in pressure at any point in a confined fluid causes an equal increase in pressure at every point within the container. This only works because liquids are incompressible – they can’t be squeezed to fit into smaller volumes. This was first realized by our friend Pascal, and is known as Pascal’s Principle.
The fact that pressure is applied equally within a uniform fluid comes in very handy when you need to lift something that is very heavy – like an automobile that needs to be repaired. Car repair garages typically have a hydraulic lift based on Pascal’s Principle.
The lift has two cylinders of different diameters, connected at their bottoms. Any pressure applied to the either side will be equally distributed throughout the liquid. So if a piston on one side is pushed down, the piston on the other side rises.
It is easy to calculate how a hydraulic lift works using the original equation for pressure, P = F/A. Pascal’s Principle tells us that the pressure on the right side equals that on the left, so we have
P_{1} = P_{2}F_{1}/A_{1} = F_{2}/A_{2}
If A_{1} = 10 cm^{2} and A_{2} = 100 cm^{2}, then the force F_{2} will be 10 times the force F_{1}
So pushing down the piston on the small, left side with a force, F_{1} of 100 N will cause a force, F_{2}, on the right side piston’s surface of 1000 N, enough to lift a car.
Of course pushing down a distance of one meter on the left, doesn’t raise the car a meter on the right. The work (W=Fd) done on one side has to equal what is done on the other:
W = F_{1}*d_{1} = F_{2}*d_{2}
so the distance piston 2 moves up is
d_{2 }= (F_{1}/ F_{2})*d_{1}
In the example above, a force of 100 N for F1, 1000 N for F2 and a distance, d1, of 1 m means the piston on the right moves up only 10 cm! This means a lot more force will be necessary to push the small piston down to make the right side go up high enough for the mechanic to get underneath the car to do repairs.
© 2013 by Wheeling Jesuit University/Center for Educational Technologies®. 316 Washington Ave., Wheeling, WV 26003-6243. All rights reserved. Privacy Policy and Terms of Use.