http://www.ck12.org Chapter 4. Motion and Forces
4.29 Pressure in Fluids
- Explain why fluids exert pressure.
- Relate pressure to force and area.
- Identify the SI unit for pressure.
- Show how to calculate pressure.
- Name factors that affect fluid pressure.
Did you ever use a bicycle pump like the one seen in the opening image? When you push down on the handle, it
forces air out through the hose, and the air enters the tire through a tiny opening. Like otherfluids(liquids and
gases), air can flow and take the shape of its container. The air that enters the tire from the pump quickly spreads out
to fill the entire tire evenly. As the tire fills with air, it feels firmer. That’s because the air exerts pressure against the
inside surface of the tire.
Why Fluids Exert Pressure
All fluids exert pressure like the air inside a tire. The particles of fluids are constantly moving in all directions
at random. As the particles move, they keep bumping into each other and into anything else in their path. These
collisions cause pressure, and the pressure is exerted equally in all directions. When particles are crowded together
in one part of an enclosed space, such as the air particles first entering a tire, they quickly spread out to fill all the
available space. That’s because particles of fluids always move from an area of higher pressure to an area of lower
pressure. This explains why air entering a tire through a tiny opening quickly fills the entire tire.