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Forces
10.27.09
8th Grade Physical Science
Ms. McKinley

What is a
Force?

a Force is a push or pull.
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2 categories : contact and non-contact
Contact Force

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exerted when two objects are touching.
Non-contact Force
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exerted on one object by another when
the objects are NOT touching.

Combining
Forces
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When 1 or more forces acts on an
object, the forces combine.
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The NET FORCE is the combo of
ALL forces acting on an object.
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When net force = 0, forces are
BALANCED.


The object does not move. Velocity
doesn’t change
When net force ≠ 0, forces are
UNBALANCED.

The velocity of the object changes
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Types of Forces:
Gravity
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An attractive force between all objects
that have mass.
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Mass: the amount of matter in an object.
Weight: the pull of gravity on an object. ALWAYS
points toward the center of the Earth.
Quick check: Which object has more mass?
Gorilla
Caterpillar
You guessed it! The Gorilla has more mass than the caterpillar
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Types of Forces:
Gravity

An attractive force between all objects that
have mass.
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Mass: the amount of matter in an object.
Gravitational force (gravity) gets
stronger as either or both objects
increase in mass.
Earth’s gravitational force is HUGE
because the Earth is HUGE!

So Earth pulls you towards itself
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Types of Forces:
Friction
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A force that resists sliding between two
touching surfaces

Friction ALWAYS works opposite the
direction of motion.
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Types of Forces:
Tension
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A pulling force exerted on an object
when it is stretched.

When you pull a rubber band, it
stretches. This is tension!

Types of Forces:
Compression

A squeezing force applied to an object
that can make an object shrink

Types of Forces:
Normal
The NORMAL FORCE
is keeping this boy
from falling through
the chair!

Force exerted by an object that is
perpendicular to the surface of the
object.
The NORMAL FORCE keeps this glass from shooting through
the table.
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Standard 2f