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Section 3: Friction and Gravity
• Objectives:
 describe friction and
identify the factors that
determine the friction force
between two surfaces
 explain how mass differs
from weight
 state the universal law of
gravitation
 describe the effects of
gravity and air resistance on
an object in free fall
I. Friction
A. Friction is a force that opposes motion.
i. strength depend:
1. type of surfaces
2. How hard surfaces push together.
B. Is friction useful?
i. Without friction motion would not be possible.
FYI: Sometimes we need to increase friction. Friction is quite often
helpful. For example, gymnasts use chalk on their hands to
increase friction. Cyclists rely on friction to hold their bicycles on
the ground during turns.
II. Types of Friction
A. Fluid Friction
B. Sliding friction
C. Rolling friction
III. Controlling Friction
A. Ways to reduce friction:
1. Smooth the surface (sanding)
2. Use lubricants. (oil, graphite, air on air hockey tables)
3. Change from sliding friction to rolling friction
B. Increase Friction:
1. Roughen up the surface.
i. adding gravel to icy roads
ii. use studded snow tires instead of radials
2. Use different type of surface.
i. carpet vs smooth tiles.
3. Increase pressure upon the surface.
IV. Gravity
A. force that pulls objects
towards Earth
FYI: Without the force of gravity, these sky-diving acrobats would simpl
float in the sky. Thanks to gravity, however, they receive a thrilling
adventure as they fall to Earth.
B. Free falli. when the only force acting on a falling object is gravity.
(Objects in free fall accelerate as it falls, because it of unbalanced
force. Unbalanced forces accelerate.)
ii. On Earth the rate of acceleration if 9.8 m/s/s
C. Projectile motion
i. curved path object follows when thrown or propelled near
surface of Earth.
ii. Projectile motion has two components: horizontal and vertical
FYI:
Although gravity pulls both a leaf and a
rock toward the earth, the two objects do
not accelerate at the same rate. The
leaf and rock do not strike the earth at
the same time. On the moon, however,
they would. Why?
D. Air resistance
i. type of fluid friction
ii. caused by object falling through air.
FYI:
Friction is the force that is
opposite to motion, so air resistance
is an upward force.
The greater the surface area of an
object, the greater the air
resistance.
Explain why the ball of paper reaches
the ground before the flat sheet.
FYI:
Without air resistance objects of
different mass will fall and
accelerate that the same rate and
land on earth at the same time.
V. Terminal Velocity
A. Terminal velocity
i. point when upward force of air resistance equals downward
pull of gravity.
FYI:
Air resistance acts on
the parachute, allowing
the parachutist to fall
at terminal velocity
that is slow enough
to allow a safe landing.
FYI:
Terminal velocity is
reached when the pull
of gravity equals the air
resistance and the falling
object no longer
accelerates.
VI. Weight
A. Weight is a measure of force of gravity
on object,
B. mass is measure of amount of matter in
that object.
C. Weight = Mass x Acc due to gravity
Example: 50 Kg person weighs
50 Kg x 9.8 m/s/s = 490 N on Earth.
VII. Universal Gravitation
A. The law of universal gravitation: force of gravity acts between all
objects in the universe.
i. amount of gravity depends on mass of object.