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Friction and Gravity
What is Friction?
• Friction is the resistance to the sliding,
rolling, or flowing motion of an object due
to its contact with another object.
• When a moving object comes in contact
with another object, friction is the opposing
force that slows or stops the moving object.
• Without friction, an object would continue to
move at constant speed forever.
• The strength of the force of friction depends on the
type of surfaces and how hard the surfaces push
together.
• Rough surfaces produce greater friction than
smooth surfaces.
Three Types of Friction
• Sliding Friction
• Rolling Friction
• Fluid Friction
Sliding Friction
•A resistance to movement that is created when
two things rub together.
Rolling Friction
When an object rolls over a surface.
Skateboard Example:
• The wheels and ball bearings in the wheels are
used to reduce friction between the floor and the
skateboard.
• The skateboard will slow and stop due to the
friction between the floor and the skateboard.
Fluid Friction
• The force that tries to slow objects down when
they move through a liquid or a gas. It's also
known as "drag", or "air resistance".
• All gases and liquids are fluids.
• An airplane and a swimmer both experience fluid
friction.
Ways to Reduce Friction
•
•
•
•
Smooth the surface
Put ball bearings in wheels
Replace rolling with sliding
Add oil or another type of lubricant
Gravity
The force that pulls objects toward each other.
Gravity and Free Fall
• An object is in free fall when the only
force acting on the object is gravity.
• Free-falling objects do not encounter
air resistance.
• In free fall, the force of gravity is an
unbalanced force.
Calculating Free Fall
All objects in free fall accelerate at the same
rate – 9.8 m/s² - regardless of their mass.
in one second = 9.8 m/s²
in two seconds = 19.6 m/s²
in three seconds = 29.4 m/s²
in four seconds = ________
The velocity continues to increase as the
object falls.
Gravity, Free Fall, and Projectile Motion
• An object that is thrown is called a
projectile.
• If there is no air resistance, an object that is
dropped and an object that is thrown are
both in free fall.
• Without the force of air resistance, both
objects will hit the ground at the same time.
Gravity and Air Resistance
• Most objects falling through air
experience a type of fluid friction
called air resistance.
• Air resistance is not the same for all
objects.
• The greater the surface area of the
object the greater the air resistance.
• However, since the elephant has
more mass, it has more downward
force of gravity and falls faster.
Universal Gravitation
The law of universal gravitation states
that the force of gravity acts between
all objects in the universe.