# When two surfaces rub the irregularities of one surface get

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```					          True or False

When two surfaces rub, the
irregularities of one surface get
caught on those of the other
surface.
True
What is friction?
Friction is the force that two
surfaces exert on each other
when the two rub against each
other.
Friction acts in a direction __ to
the objects direction of motion.
Opposite
The strength of the force of
friction depends on what two
factors?
The types of surfaces involved
and how hard the surfaces push
together.
How is friction involved in
sledding and skiing?
There is very little friction
between a sled or skis and the
snow because the snow is
relatively smooth.

Also, the snow may be partially
melted, and fluid friction is much
easier to overcome.
Why does some friction occur
with seemingly smooth surfaces?
Even surfaces that seem smooth
have irregular bumps that you
could see with a microscope, so
friction occurs even with
smooth-looking objects.
Which kind of friction?

Friction occurs when an object
moves through a liquid or gas
Fluid friction
Which kind of friction?

Friction occurs when solid
surfaces slide over each other
Sliding friction
Which kind of friction?

Friction occurs when an object
rolls over a surface
Rolling friction
Which kind of friction?

Friction occurs when objects
are not moving
Static friction
Which kind of friction requires
more force to overcome, rolling
friction or sliding friction?
Sliding friction
What kind of friction occurs when
moving parts have ball bearings?
Rolling friction
How does oil between
machine parts reduce friction?
The oil keeps the machine parts
from making direct contact, and
there is fluid friction between the
A force that pulls objects
toward each other is called __.
gravity
True or False

The force that makes an apple
fall to the ground is the same
force that keeps Earth orbiting
the sun.
True
The force of attraction between
two objects varies with what two
factors?
Mass and distance between
the objects.
How is weight different from
mass?
Weight is a measure of the force
of gravity on an object, while
mass is a measure of the amount
of matter that makes up an
object.
Weight is usually measured in
__.
newtons
True or False

On the moon, your mass would
be less than it is on Earth, but
your weight would be the same.
False
When is an object said to be
in free fall?
An object is in free fall when the
only force acting on the object is
gravity.
Near the surface of Earth, what is
the acceleration of an object due
to the force of gravity?
9.8 m/s2
Objects falling through air
experience a type of fluid friction
called __.
Air resistance
The greatest velocity a falling
object reaches is called __.
Terminal velocity
True or False

The greater the surface area of
an object, the greater the air
resistance.
True
An object that is thrown is
called a(n) __.
projectile
True or False

An object that is dropped will hit
the ground before an object that
is thrown horizontally from the
same height.
False
Draw arrows that show
the forces acting on the
falling coconut.

Label each arrow with
the NAME of the force.

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 views: 39 posted: 3/27/2011 language: English pages: 54