13.0 - Introduction
The study of physics typically begins with the study of
solid objects: You learn how to determine the velocity
of a car as it accelerates down a street, what happens
when two pool balls collide, and so on.
This chapter introduces the study of fluids. Liquids
and gases are both fluids. Fluids change shape much
more readily than solids. Pour soda from a can into a
glass and the liquid will change shape to conform to
the shape of the glass. Push on a balloon full of air or
water and you can easily change the shape of the
balloon and the fluid it contains. In contrast to liquids,
gases expand to fill all the space available to them.
One reason astronauts wear spacesuits is to keep
their air near them, and not let it expand limitlessly into the near vacuum of space.
This chapter focuses on the characteristics exhibited by fluids when their temperature and density remain nearly constant. It covers topics such
as the method of calculating how much pressure water will exert on a submerged submarine, and why a boat floats. Some of the topics apply
to liquids alone, while others apply to both liquids and gases.
The glass and the ice cube in this photo are solid objects. The
water is a liquid. The bubbles and the surrounding air are gasses.13.1 - Fluid
Fluid: A substance that can flow and conform
to the shape of a container. Liquids and gases
are fluids.
A fluid alters its shape to conform to the shape of the container that surrounds and
holds it. The molecules of a fluid can “flow” because they are not fixed into position as
they would be in a solid. Liquids and gases are fluids, and they are two of the common
forms of matter, with solids being the third. There are other forms of matter as well,
such as plasma (created in fusion reactors) and degenerate matter (found in neutron
stars).
A substance can exist as a solid, a liquid or a gas depending on the surrounding
physical conditions. Factors such as temperature and pressure determine its state. For
the purposes of a physics textbook, we need to be specific about what state of matter
we are discussing at any given time. However, whether a substance is considered a
solid or a fluid may depend on factors such as the time scale under consideration. For
instance, the ice in a glacier can seem quite solid, but glaciers do flow slowly over time,
so treating glacier ice as a fluid is useful to geologists.
Fluids
Can flow
Rate of flow variesFluids
Conform to container