Download Chapter 12.2

Chapter 12, Section 2
Key Concept: Friction is a force that opposes motion.
BEFORE, you learned
• Gravity is the attractive force
masses exert on each other
• Gravity increases with greater
mass and decreases with
greater distance
• Gravity is the centripetal force
keeping objects in orbit
NOW, you will learn
• How friction affects motion
• About factors that affect
friction
• About air resistance
.
THINK ABOUT
What forces help you to walk?
As a person walks, she exerts a backward force on the ground.
A reaction force moves her forward. But some surfaces are harder to walk
on than others. Ice, for example, is harder to walk on than a dry surface
because ice is slippery. How can different surfaces affect your ability to walk?
Friction occurs when surfaces slide against each other.
Have you ever pushed a heavy box across the floor? You probably
noticed that it is easier to push the box over some surfaces than
over others. You must apply a certain amount of force to the box
to keep it moving. The force that acts against your pushing force is
called friction. Friction is a force that resists the motion between two
surfaces in contact.
When you try to slide two surfaces across each other, the force of
friction resists the sliding motion. If there were no friction, the box
would move as soon as you applied any force to it. Although friction
can make some tasks more difficult, most activities, including walking,
would be impossible without it. Friction between your feet and the
ground is what provides the action and reaction forces that enable
you to walk.
Forces and Surfaces
If you look down from a great height, such as from the window of an
airplane, a flat field appears to be smooth. If you were to walk in the
field, however, you would see that the ground has many bumps and
holes. In the same way, a flat surface such as a piece of plastic may
look and feel smooth. However, if you look at the plastic through a
strong microscope, you see that it has tiny bumps and ridges. Friction
depends on how these bumps and ridges on one surface interact with
and stick to the bumps and ridges on other surfaces. There are several
factors that determine the friction between two surfaces.
 Types of Surfaces Friction between two surfaces depends on the
materials that make up the surfaces. Different combinations of
surfaces produce different frictional forces. A rubber hockey puck
sliding across ice has a smaller frictional force on it than the same
puck sliding across a wooden floor. The friction between rubber and
ice is less than the friction between rubber and wood.
 Motion of the Surfaces You need a larger force to start something
moving than you do to keep something moving. If you have ever
tried to push a heavy chair, you may have noticed that you had to
push harder and harder until the chair suddenly accelerated forward.
As you apply a force to push a chair or any other object that is not
moving, the frictional force keeping it from sliding increases so the
forces stay balanced. However, the frictional force has a limit to how
large it can be. When your force is greater than this limit, the forces on
the chair are no longer balanced, and the chair moves. The frictional
force remains at a new lower level once the chair is moving.
 Force Pressing the Surfaces Together The harder two surfaces
are pushed together, the more difficult it is for the surfaces to slide
over each other. When an object is placed on a surface, the weight of the
object presses on that surface. The surface exerts an equal and opposite
reaction force on the object. This reaction force is one of the
factors that determines how much friction there is.
If you push a chair across the floor, there will be a certain amount
of friction between the chair and the floor. Increasing the weight of
the chair increases the force pushing the surfaces together. The force
of friction between the chair and the floor is greater when a person is
sitting in it than when the chair was empty.
Friction depends on the total force pressing the surfaces together,
not on how much area this force acts over. Consider a rectangular
cardboard box. It can rest with its smaller or larger side on the floor.
The box will have the same force from friction regardless of which
side sits on the floor. The larger side has more area in contact with
the floor than the smaller side, but the weight of the box is more
spread out on the larger side.
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 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.
When an object moves through a fluid, it pushes the
molecules of the fluid out of the way. At the same time, the
molecules of the fluid exert an equal and opposite force on the
object that slows it down. This force resisting motion through a
fluid is a type of friction that is often called drag. Friction in
fluids depends on the shape of the moving object. Objects can
be designed either to increase or reduce the friction caused by a
fluid. Airplane designs, for example, improve as engineers find
ways to reduce drag.
The friction due to air is often called air resistance.
Air resistance differs from the friction between solid surfaces.
Air resistance depends on surface area and the speed of an
object in the following ways:
 An object with a larger surface area comes into contact with
more molecules as it moves than an object with a smaller surface
area. This increases the air resistance.
 The faster an object moves through air, the more molecules it
comes into contact with in a given amount of time. As the speed
of the object increases, air resistance increases.
When a skydiver jumps out of a plane, gravity causes the skydiver
to accelerate toward the ground. As the skydiver falls, his body pushes
against the air. The air pushes back—with the force of air resistance.
As the skydiver’s speed increases, his air resistance increases. Eventually,
air resistance balances gravity, and the skydiver reaches terminal velocity,
which is the final, maximum velocity of a falling object. When the
skydiver opens his parachute, air resistance increases still further, and
he reaches a new, slower terminal velocity that enables him to land safely.
check y
our
reading How do speed and surface area affect air resistance?
Questions for Chapter 12, Section 2
KEY CONCEPTS
1. How does friction affect forward motion? Give an example.
2. Describe two ways to change the frictional force between
two solid surfaces.
3. How does air resistance affect the velocity of a falling object?
CRITICAL THINKING
4. Infer What two sources of friction do you have to overcome
when you are walking?
5. Synthesize If you push a chair across the floor at a
constant velocity, how does the force of friction compare
with the force you exert? Explain.
CHALLENGE
6. Synthesize If you push a book against a wall hard enough, it will not
slide down even though gravity is pulling it. Use what you know
about friction and Newton’s laws of motion to explain why the
book does not fall.