Friction and Heat
Friction between surfaces produces heat. You feel heat produced by
friction when you rub your hands together. As you rub, friction causes
the individual molecules on the surface of your hands to move faster.
As the individual molecules in an object move faster, the temperature
of the object increases. The increased speed of the molecules on the
surface of your hands produces the warmth that you feel.
The heat produced by friction can be intense. The friction that
results from striking a match against a rough surface produces enough
heat to ignite the flammable substance on the head of the match. In
some machines, such as a car engine, too much heat from friction can
cause serious damage. Substances such as oil are often used to reduce
friction between moving parts in machines. Without motor oil, a car’s
engine parts would overheat and stop working.
Friction produces sparks
between a match head and
a rough surface. The heat
from friction eventually
lights the match.
Motion through fluids produces friction.
As you have seen, two objects falling in a vacuum fall with the same
acceleration. Objects falling through air, however, have different accelerations. This difference occurs because air is a fluid. A fluid is a
substance that can flow easily. Gases and liquids are fluids.
Friction in Air
How does the shape of an object
affect how it falls?
Write a hypothesis that explains how shape affects the speed
of falling objects. Design an experiment that tests your hypothesis.
PROCEDURE
1 Figure out how you can use the three sheets of paper to test your hypothesis.
Remember to control all other variables, including the mass of the paper.
2 Write up your procedure.
3 Conduct your experiment.
WHAT DO YOU THINK?
• What were the results of your experiment?
• Did the results support your hypothesis? Explain your answer.
• Write a statement that summarizes your findings.
CHALLENGE What other variable might affect falling time? How could
you test it?
408 Unit 3: Motion and Forces
SKILL FOCUS
Designing
experiments
MATERIALS
3 identical sheets
of paper
TIME
30 minutes