s8pe-10303-ca
12/8/05
6:09 PM
MAZER
Page 89
Friction
How does the friction created by different
surfaces compare?
SKILL FOCUS
Measuring (8.2.a)
PROCEDURE
1 Hook the spring scale to the mass and slowly but steadily pull the mass across
the flat surface of your desk or worktable. Measure and record the force on
the mass as you pulled it across the desk.
2 Tape a sheet of sandpaper on a flat surface. Pull the mass across the sand-
paper. Measure and record the force on the mass as you pull steadily.
3 Repeat step 2 using aluminum foil, and again using the sheet of paper.
WHAT DO YOU THINK?
• Which surface caused the mass to have the greatest force? the least force?
• What is the relationship between the amount of force you measured on the
spring scale and the amount of friction created by each surface?
MATERIALS
• 500-g hooked
mass
• spring scale
• medium grain
sandpaper
• aluminum foil
• sheet of paper
• masking tape
TIME
25 minutes
CHALLENGE Watch the spring scale as you pull on the mass
before it starts moving. What is the maximum force you apply
before the mass moves? How does this force compare to the
force you measured after the mass was moving?
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.
Motion through fluids produces friction.
As you have seen, two objects in a vacuum fall with the same
acceleration. Objects falling through air, however, may have different
accelerations. This difference occurs because air causes friction. Air is
a fluid. A fluid is a substance that can flow easily. Gases and liquids
are fluids.
Friction produces sparks
between a match head
and a rough surface. The
heat from friction lights
the match.
Chapter 3: Gravity, Elastic Forces, and Friction 89
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