http://www.ck12.org Chapter 4. Motion and Forces
Pascal and His Law
Some of the earliest scientific research on pressure in fluids was conducted by a French mathematician and physicist
named Blaise Pascal (1623–1662). The SI unit of pressure, the Pascal (Pa), is named for him because of his important
research. One of Pascal’s major contributions is known asPascal’s law. This law states that a change in pressure at
any point in an enclosed fluid is transmitted equally throughout the fluid. To see a video about Pascal’s law, go to
this URL:
http://www.youtube.com/watch?v=4uRnPTQxZtw
MEDIA
Click image to the left for use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/5027A Simple Example
A simple example may help you understand Pascal’s law. Toothpaste is a fluid that is enclosed in a tube with a small
opening at one end. Look at the toothpaste tube in theFigure4.62. When any part of the tube is squeezed, toothpaste
squirts out the open end. The pressure applied to the tube is transmitted equally throughout the toothpaste. When
the pressure reaches the open end, it forces toothpaste out through the opening.
FIGURE 4.62
Using Fluid Pressure
The ability of fluids to transmit pressure in this way can be very useful—besides getting toothpaste out of a tube!
For example, hydraulic brakes in a car use fluid to transmit pressure, and when they do, they also increase force.
You can see how hydraulic brakes work in theFigure4.63.
Q: In this diagram, how can you tell that the force applied by the brake cylinder is greater than the force applied by
the brake pedal mechanism?
A: The arrows representing the force applied by the break cylinder are larger than the arrow representing the force
applied by the brake pedal mechanism. A larger arrow indicates greater force.
Q: How do hydraulic brakes increase the force that is applied to the brake shoes?