Chapter 10
Forces
8th Grade Science
Force
• _______a push or a pull
Force
• One object exerts a _______
on another
Force
• Force is described by its ______
strength and by the
direction
________
in which it acts
• Example – if you ______
on a door, you exert a
push
Force in a different _______
direction than if you ____
_____
pull
on the door.
Static Friction
• _____________
- the friction that acts on an
Static Friction
object that is not moving.
• You must use________
Extra Force to start motion of a
stationary object.
• If you use a force that is less than force of
___________,
then the object that is
Static Friction
stationary will not move.
• Example: pushing a desk on the floor
Sliding / Rolling Friction
• ____________
Sliding Friction - occurs when two surfaces slide
over each other.
• Example: a ballet dancer puts sticky powder on
the bottom of ballet shoes so that they do not
slip on the floor.
Rolling Friction
• _____________
- when an object rolls across a
surface
• Example: skateboard – rolling friction acts in the
_________
direction of the skateboards motion.
opposite
Fluid Friction
• ___________
- occurs when a solid object
Fluid Friction
moves through a liquid.
• Example: parts of a machine are bathed in oil
so that they move smoothly over each other.
• Example 2: a bike rider experiences fluid
friction between the individual and the air.
Pushed fluid aside fluid friction
• When an object ____________,
occurs
Gravity
• _______
Gravity - a force that pulls objects towards
each other
Isaac Newton - concluded that force acts to pull
• __________
objects straight downward towards the center
of the Earth.
• Newton realized that ________
acts
gravity
everywhere in the _______,
not just on Earth
universe
= ___________________
Law of Universal Gravitation
Law of Universal Gravitation
• Law of Universal Gravitation states that
The
force of gravity acts between all objects in the universe
_____________________________________
• Any two objects in the universe,
Without Exception
_______________,
attract each other.
• We are attracted to the _______
and all the other
Earth
objects around us.
• Objects all around us are _________
to us as well
attracted
– we don’t know it because the forces are small
compared to the force of Earth’s attraction.
Factors Affecting Gravity
• Two factors that affect gravitational attraction
between two objects are mass
____ and ________
distance
• The more _____
mass an object has, the greater the
Gravitational force
______________.
Sun’s
• Because the ______
mass is so great, it exerts
a large gravitational force on the ________
planets
distance
• ________
- the farther the two objects are,
the _________
the gravitational force
lesser
between the objects
Weight and Mass
• ________
- the measure of the amount of
mass
matter in an object
• _________
- the measure of the gravitational
weight
force exerted on an object
• When you step on a scale, you are
determining the amount of gravitational force
the Earth is exerting on you!
• _________
varies but _______
does not
Weight
Mass
Gravity and Motion
• On Earth, gravity is a ________
downward force that
affects all
__ objects.
• __________
- when the force of gravity is an
Free Fall
____________
force, which causes the object
Unbalanced
accelerate
to _________.
• Near the surface of the Earth, the acceleration
due to gravity is 9.8
_____
m/s - regardless of the
object’s ______.
mass
Math / Analyzing the Data
• Page 346 (1-4)
Air Resistance
Air Resistance
• ____________
- fluid friction that objects
encounter while falling through the air.
upward
• Air resistance is an _______
force that is
exerted on an object falling _________.
downward
• Falling objects with a greater ____________
Surface area
experience ______air
resistance than objects
more
with less surface area.
• Air resistance increases with ________
velocity
Air Resistance Continued
• As a falling object gains speed,
______________
increases also – eventually
Air resistance
the velocity of the falling object will be fast
enough that the ________
force of air
upward
resistance is equal to the ________
downward force of
Balanced force
gravity = _______________
• When forces are ________
there is no
balanced
_________
acceleration - velocity remains constant
Projectile Motion
Terminal Velocity
• __________________
- the greatest velocity
of a falling object
• ________
projectile - an object that is thrown
• When a projectile is thrown at an ________
upward
reduces
angle, the force of gravity ________
its
vertical velocity.
• Eventually the ______
upward motion of the projectile
will stop and ________
will pull it ________
gravity
downward
Section Assessment
• Page 348 (1-3)
Newton’s Laws
• Newton proposed three basic laws of motion
in the 1600’s
• Newton’s first law restated Galileo’s
_________ idea
about force and motion.
• Newton’s first law of motion states: An object
at rest will remain at rest, and an object
moving at a constant velocity will continue
moving at a constant velocity, unless it is acted
upon by an unbalanced force.
1st Law
• An object that is not moving, will not move
until a ___________.
Force acts on it
• If an object is moving, it will continue to move
at a constant velocity until ________________
force acts to either
_________________________
Change either speed or direction
On Earth, ________
and _________
are
gravity
friction
_______________
Unbalanced forces that often change an objects
motion.
Inertia
inertia
• _______
- is the tendency of an object to
resist a change in motion - ______________
Galileo’s concept
• Newton’s first law of motion is also called the
_________.
Law of inertia
• Inertia explains why a person moves forward
in their seat when a car stops suddenly.
________
moves you forward in your seat
inertia
while a seat belt ________________
Changes your direction
Inertia depends on Mass
greater
mass
• The _________
the _______
of an object, the
__________
its inertia
______ and the _________
greater
greater
force
Change its motion
the ________
required to ______________.
Second Law of Motion
acceleration depends on an objects mass
• _________
________
and the ____________
acting on the object.
Net force
• Acceleration = Net Force
Mass
Acceleration
Meters per second²
• Acceleration is measured in _______________
• Mass is measured in __________
Kilograms (kg)
• According to Newton’s Second Law – force is
kg times m/s² = _____________
Newton (N)
measured in __________
• ________
= SI unit of force
Newton
• Math Sample Problem on page 351
• Math Practice page 351 (1 and 2)
Newton’s Third Law
• To increase __________
you must either
acceleration
mass
increase _____
force or decrease _____
• Newton’s Third Law states: if one object exerts
a ________
on another object, then the
force
second object exerts a ________
of
force
_____________
in the ____________
Equal strength
Opposite direction on the
first object.
• FOR EVERY ACTION THERE IS AN EQUAL BUT
OPPOSITE REACTION
Why don’t action and reaction forces
cancel each other?
• Read Examples on pages 354-355
Momentum
• ____________
is a characteristic of a moving
momentum
object that is related to the mass and velocity
of the object
• Momentum of a moving object is calculated
by __________________________
Multiplying mass times velocity (m x v)
• The more momentum a moving object has,
the more difficult its is to stop
• Objects with greater ______
and ________
mass
velocity
have greater ____________
momentum
Conservation of Momentum
conservation
• ____________
- conditions before and after
an event
• An amount that is conserved is the same
amount ______
the event as it was ________
after
before
• The total amount of momentum objects have
is _________
when they ________
conserved
collide
• Momentum can be __________
but not lost
____
transferred
Law of Conservation of Momentum
• _____________
Total Momentum of any group of objects
remains the same, or is ___________,
unless
conserved
Outside forces act on the objects.
___________
• Examples on page 358-359 - three different
types of collisions
Section Assessment
• Page 359 (1-5)