15.2. Buoyancy of Fluids http://www.ck12.org
Because of buoyant force, objects seem lighter in water. You may have noticed this when you went swimming and
could easily pick up a friend or sibling under the water. Some of the person’s weight was countered by the buoyant
force of the water.
Density and Buoyant Force
Density, or the amount of mass in a given volume, is also related to buoyancy. That’s because density affects weight.
A given volume of a denser substance is heavier than the same volume of a less dense substance. For example, ice
is less dense than liquid water. This explains why ice cubes float in a glass of water. This and other examples of
density and buoyant force are illustrated inFigure15.14 and in the video at this URL: http://www.youtube.com/w
atch?v=VDSYXmvjg6M (12:08).
MEDIA
Click image to the left for use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/5028Archimedes’ Law
Did you ever notice that when you get into a bathtub of water the level of the water rises? More than 2200 years ago,
a Greek mathematician named Archimedes noticed the same thing. He observed that both a body and the water in a
tub can’t occupy the same space at the same time. As a result, some of the water is displaced, or moved out of the
way. How much water is displaced? Archimedes determined that the volume of displaced water equals the volume
of the submerged object. So more water is displaced by a bigger body than a smaller one.
What does displacement have to do with buoyant force? Everything! Archimedes discovered that the buoyant force
acting on an object in a fluid equals the weight of the fluid displaced by the object. This is known asArchimedes’
law(or Archimedes’ Principle).
Archimedes’ law explains why some objects float in fluids even though they are very heavy. Remember the oil tanker
that opened this chapter? It is extremely heavy, yet it stays afloat. If a steel ball with the same weight as the ship
were put into water, it would sink to the bottom (seeFigure15.15). That’s because the volume of water displaced
by the steel ball weighs less than the ball. As a result, the buoyant force is not as great as the force of gravity acting
on the ball. The design of the ship’s hull, on the other hand, causes it to displace much more water than the ball. In
fact, the weight of the displaced water is greater than the weight of the ship, so the buoyant force is greater than the
force of gravity acting on the ship. As a result, the ship floats. You can check your understanding of Archimedes’
law by doing the brainteaser at this URL: http://www.pbs.org/wgbh/nova/lasalle/buoyquestion.html.
For an entertaining video presentation of Archimedes’ law, go to this URL: http://videos.howstuffworks.com/disc
overy/6540-mythbusters-lets-talk-buoyancy-video.htm.
Lesson Summary
- Buoyancy is the ability of a fluid to exert an upward force on any object placed in it. The upward force is
called buoyant force. An object’s weight and the buoyant force acting on it determine whether the object sinks