### File - 8th Grade Physical Science

```FORCES
Types of Friction, Newton’s Laws of Motion,
Gravity, and Types of Forces
FRICTION
• Friction is all around us. Caused by the
minor bumps and jagged edges in all
surfaces there is no way for humans to
escape it!
• There are 4 types of friction: static, sliding,
rolling and fluid. Each of these have their
ENERGIZER
• In your own words, write a definition of the word
friction. What is one way that friction can be
increased?
Friction: A Force Opposing Motion
• Friction is a force that acts in a direction
opposite to the motion of the moving object
causing the moving object to slow down and
finally stop.
• Friction arises from the fact that objects and
surfaces are not perfectly smooth.
• The amount of friction between two surfaces
depends on how hard the surfaces are forced
together and on the materials of which the
Sliding friction
• When solid objects slide over each other,
they produce sliding friction. In order to
move the object you must exert a force
larger than the force of friction.
• The amount of sliding friction depends on:
• weight of the moving object.
• type of surface that the object is sliding
across.
Rolling friction
• When an object rolls over a surface rolling friction
is produced
• Produced by objects such as wheels and ball
bearings
• it tends to be less than that of sliding friction.
Fluid friction
• As an object moves
through a fluid it
produces fluid friction
• fluid friction is less
than sliding friction.
• Air resistance is an
example of fluid friction
caused by the particles
that make up air.
• All liquids and gases are
fluids
• Friction also exists
when an object moves
across or through a
fluid.
• Substance called
lubricants (oil, wax,
and grease), change
sliding friction to fluid
friction, thus reducing
friction.
Static Friction
• When a force is
applied to an object
but does not cause it
to move.
• The force of static
friction balances the
applied force.
• Static friction
disappears as soon as
an object begins to
move.
Friction: Is it helpful or harmful?
• An example:
• In your car friction can be both
helpful and harmful, here is how:
• When you press on the breaks you
cause friction between them which
• In this same car there is friction
between the moving parts in the
engine. This friction causes the
parts to wear out over time and
therefore they must be replaced.
• Friction between your foot and
shoe can make holes in your
socks!
• Friction between your shoe and the
floor helps you to stand upright.
• Friction is not always a problem . . . as a matter of fact, in
many cases it can be helpful.
•How might we lessen the amount of slide friction?
ENERGIZER
• Give an example of how
friction can be harmful
when running a car. Then
give and example of how it
identify the type of friction
Newton’s Laws of Motion
Newton’s Laws of Motion
• Research Newton’s 2nd and 3rd Laws of Motion.
• Create a poster, skit, song or poem about each law.
• A poster should include a definition, picture, formulas as
appropriate and color
• Skits should include a definition and a real life demonstration
of each law
• Songs and poems should include a definition and real life
example of each law.
• Have fun, be creative and be ready to present
tomorrow!
Newton’s First Law of Motion
or
The Law of Inertia
An object at rest will remain at rest
and
an object in motion will remain in motion
at a constant velocity
unless acted on by an unbalanced force.
Newton’s Second Law
• The acceleration of an object depends on the mass of the
object and the amount of force applied.
•F=mxa
Part 1
• Acceleration Depends on Mass
• More force is needed to accelerate objects that have more mass.
• If the same force is applied to an object with a large mass vs. a
small mass the smaller object will go farther because it is easier to
accelerate.
40 N
40 N
Part 2
• Acceleration Depends on Force
• If you put more force on the same size object it will move faster.
• An objects acceleration increases as the force on the object increases.
• F=mxa
Newton’s Third Law
• Whenever one object exerts a force on another object, the
second object exerts and equal and opposite force on the
first.
• All forces act in pairs.
• When a force is exerted and equal and opposite force occurs (sitting in
a chair).
• Without action/reaction forces there would be no motion.
• These 2 forces act on different objects.
Action-reaction force pairs make it possible for fish (or
humans) to swim.
• A fish uses its fins to push water backwards. But a push
on the water will only serve to accelerate the water. In
turn, the water reacts by pushing the fish forwards,
propelling the fish through the water. The size of the force
on the water equals the size of the force on the fish; the
direction of the force on the water (backwards) is opposite
to the direction of the force on the fish (forwards).
Types of Forces
Contact forces are types of
forces in which the two
interacting objects are physically
contacting each other.
• Applied Force
• Frictional Force
• Tensional Force
• Normal Force
• Air Resistance Force (which is
a type of frictional force)
• Spring Force
Action-at-a-distance forces are
types of forces in which the two
interacting objects are not in
physical contact with each other,
yet are able to exert a push or
pull despite a physical
separation.
•
•
•
•
Gravitational Force
Electrical Force
Magnetic Force
Centripetal Force
APPLIED FORCE – PUSH OR PULL
FRICTIONAL FORCE – GOES AGAINST
THE DIRECTION OF THE MOTION
TENSIONAL FORCE –WHEN THINGS
ARE PULLING IN OPPOSITE
DIRECTION
NORMAL FORCE – OPPOSES THE
GROUND
AIR RESISTANCE FORCE – TYPE
OF FRICTIONAL FORCE
SPRING FORCE
Recognizing Forces
Choose from these
forces
• Applied Force
• Frictional Force
• Tensional Force
• Normal Force
• Air Resistance Force
• Spring Force
• Gravitational Force
• A skydiver (who hasn't
opened his parachute yet)
falls at terminal velocity.
Consider the forces acting
on the skydiver.
• A block rests on top of a
table. Consider only the
forces acting upon the block.
• A block slides across the top
of a table. Consider only the
forces acting upon the block.
Recognizing Forces
Choose from these
forces
• Applied Force
• Frictional Force
• Tensional Force
• Normal Force
• Air Resistance Force
• Spring Force
• Gravitational Force
• A block hangs at rest from
the ceiling by a piece of
rope. Consider the forces
acting on the block.
• A ball is shot into the air with
Consider the forces acting
on the ball while it is in the
air.
• The wheels of a car are
locked as it skids to a stop
while moving across a level
highway.
• A car is attached by a cable
to a moving truck and is
being pulled along a level
highway. Consider only the
forces acting on the car.
Force Diagram
Force Diagrams
Applied Force
Force of
Gravity
Normal
Force
Force of Friction
ENERGIZER
ANSWER TODAY’S EQ IN A FEW
WHEN YOU ARE SKYDIVING ARE YOU IN
FREE FALL?
Gravity Myth or Fact??
1. Gravity exists only on Earth. (Myth)
(Fact) Gravity also affects objects in space. In fact,
objects stay in orbit because of gravity. Without
gravity, a satellite launched from the Earth would
simply drift off endlessly into space, traveling in a
straight line, instead of circling the planet. Gravity
pulls objects toward the center of the planet,
causing them to accelerate and drop toward the
planet
Gravity Myth or Fact??
2. Weight and mass are the same. (Myth)
(Fact) Weight and mass are not the same. Mass is a
measure of a body's resistance to changes in its state of
motion, which depends on the amount of matter it
contains. Weight is the force of gravity exerted on a body
due to its mass and its location near another, more
massive object. An astronaut in space can be weightless
but cannot be without mass.
Gravity Clip
• How does this clip tie into gravity and relate to the
Energizer and EQ with sky diving?
Why does an apple fall towards the
ground when dropped?
• Isaac Newton asked this question in 1665.
• The answer to this question gave birth to a law…
The Law of Universal Gravitation
• All objects in the universe attract each other through
gravitational force.
• The size of this force depends on the masses of the
objects and the distance between them.
• So…
• Gravitational force increases as mass increases.
• Gravitational force decreases as distance increases.
How Mass Affects Gravitational Force:
How Distance Affects Gravitational Force:
• On Earth, all objects
experience a "downward"
force of gravity.
• The force of gravity on an
object is always equal to
the weight of the object.
• Gravity is of great
importance in the
interaction of all objects in
the universe;
• it binds us to the Earth,
• holds the Earth and all
planets in the solar system,
Gravity is what holds
the universe together
Gravity in Motion
• All objects fall to the ground at the same RATE
because the acceleration due to gravity is the same
for all objects.
• This is because heavier objects experience more
gravitational pull but are also harder to accelerate than a
lighter object.
Acceleration Due to Gravity
• ALL objects accelerate toward earth at a rate of 9.8
m/s/s.
• How do we calculate an objects change in velocity
during free fall?
• Change in velocity= g x t
• g=acceleration due to gravity
• t=time
VELOCITY DUE TO GRAVITY
PROBLEM
•A boy standing on a high cliff dives into the ocean
below and strikes the water after 3 seconds. What is
the boy’s velocity when he hits the water?
• V=gxt
• V = 9.8 m/s/s x 3 s
• V = 29.4 m/s downward
One more
• A foul ball is hit straight up in the air and falls from
the top of its motion for 1.4 seconds before being
caught by the catcher. What is the velocity of the
ball as it hits the catcher’s glove?
• V=g x t
• V=9.8 m/s/s x 1.4 s
• V= 13.72 m/s downward
A few more things on acceleration due to
gravity
• All objects fall to the ground at the same rate; HOWEVER,
differences in the size, shape and speed of an object can
increase or decrease air resistance.
• As speed increases so does air resistance
• At a certain point the two will equal out and the object will fall at its
terminal velocity (a constant rate).
Quick Quiz
1.
2.
3.
What is the difference between mass and
weight?
What must you know in order to calculate the
gravitational force between two objects?
Name three common items that could be used to
increase friction.
Weight
Weight is
• a measure of the force of
gravity on an object
• Metric or SI unit is
the Newton (N)
according to the force of
gravity pulling on you
•The force of gravity
varies according to the
distance the object is
from the center of the
Earth.
- You weigh less on top of
a mountain than you do at
sea level.
ENERGIZER
• A girl goes to the doctor and finds that she weighs
95 pounds. What is her weight in Newtons?
• M = lb / 2.2
• M = 95/2.2
• M = 43.18 kg
• W = kg*9.8
• W = 43.18*9.8
• W = 423.16 N
Free Fall
Terminal Velocity
Air
Resistance
Gravity
gravity
Projectile Motion:
Horizontal motion (inertia)
Gravity
Actual path of
baseball
ORBITING OBJECTS INVESTIGATION
• Swing the stopper on a string around in a circle
observing all of the forces acting on the stopper.
(Make sure to put space between you and your
neighbor.)
• Draw a picture of the stopper being swung in a
circle.
• Label all forces with direction acting on the stopper
from the moment it begins to swing.
Orbital Motion:
Forward motion (inertia) of the
space shuttle
Gravity
Centripetal Force
• Centripetal acceleration: acceleration toward the center of
a curved or circular path
• When the net force is toward the center of the curved path
this is called centripetal force.
• Gravity can be considered a centripetal force when
objects are in orbital motion.
• Weight (N) = Mass (kg) x 9.8
• Your mass (kg) can be found in two ways….
• Mass = your pounds ÷ 2.2
• There are 2.2 lbs in 1 kg
• Mass = your pounds x 0.4545
• There is about 0.4545 kg in 1 pound
ENERGIZER
If Wile E. Coyote and a boulder fall off a cliff at the
same time, which do you think will hit the ground
first?
Label the forces
involved in Tom