Chapter 2, Section 2
Friction and Gravity
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Friction = the force that two surfaces put on
each other when rubbed together.
The amount of friction depends upon the
type of surface and how much force is used
when rubbing the objects together.
◦ Ex. – rubbing hands together
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Static Friction – friction on objects that are
not moving.
◦ Moving a heavy desk across the floor
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Sliding Friction – friction when two solid
surfaces slide over each other
◦ Brakes on a bicycle tire
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Rolling Friction – when an object rolls across
the surface.
◦ Ball bearing between the wheels and a skateboard
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Fluid Friction – when a solid object moves
through a liquid (water, oil, or air)
◦ Surfer on the water
Types of Friction
p. 45 in textbook
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Gravity – a force that pulls objects toward
each other
Gravity pulls objects straight down toward
Earth’s center.
Gravity is everywhere.
Sometimes also called universal gravitation.
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Mass and Distance
Mass – the amount of matter in an object.
◦ The more the mass, the greater the gravitational
force.
◦ The farther apart two objects are, the less the
gravitational force.
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Weight – a measure of the gravitational force
exerted on an object.
◦ Weight will depend upon the strength of the
gravitational pull. (Earth vs. Moon)
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Free Fall – when the only force acting on an
object is gravity.
◦ Gravity becomes an unbalanced force allowing the
object to move.
◦ Near the surface of earth, acceleration from gravity
is 9.8 m/s2.
◦ All objects in free fall accelerate at the same rate no
matter what their mass in the absence of air.
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Air Resistance – objects falling through air
experience this fluid friction.
◦ The larger the surface area, the greater the air
resistance.
◦ As velocity increases, so does the air resistance.
◦ Terminal velocity = when the force of air resistance
equals the weight of the object.
 The falling object falls fast enough, the upward force
of air resistance becomes equal to the downward force
of gravity. The forces become balanced.
 The object continues to fall but velocity remains
constant.