Download Forces all around us - 8th Grade Physical Science

WARM UP – 11/13/12

How would you define the term force? What does
a force do?
FORCES FOLDABLE: TITLE EACH FLAP AND DRAW A
PICTURE, THEN INCLUDE ALL INFORMATION LISTED BELOW
What is a Force? (title)
*Define:
-Force
-Newton
-Net Force
Balanced Forces (title)
Forces in the SAME direction vs.
Forces in OPPOSITE direction
(title)
*Define Each
*How is net force
determined?
*Draw a diagram with arrows
for each.
Unbalanced Forces (title)
*Define
*Define
*Draw a Picture using arrows to
*Draw a Picture using arrows to
show force size and direction
show force size and direction
Forces all around us
The Nature of Force
• A force gives energy to an object, sometimes
causing it to start moving, stop moving, or
change direction.
• A force can be a push or pull.
• Newton: the unit used to express force (I
pushed the chair with 50 N of force)
• Net Force: the combination of all forces acting
on an object
Forces Acting in the Same Direction

When multiple forces act upon an object in the
same direction, you ADD the forces to find the net
force acting on the object.
Forces Acting in Opposite Directions

When multiple forces act upon an object in the
opposite directions, you SUBTRACT the forces to
find the net force acting on the object.
Balanced Forces

Balanced forces cause NO
CHANGE in motion

Forces that are in opposite
directions and equal in size
are called balanced forces.

When forces are balanced,
there is no change in motion;
there would be no overall
force (net force) acting on the
object.
Balanced Forces
Unbalanced Forces

Unbalanced forces cause a
CHANGE in motion
Same direction
 When two forces are acting
in the same direction, they
are added together.
 The size of the arrow is
relative to the strength of
the force.
•This force is
unbalanced because
there is a change in
motion
Unbalanced Forces
Same Direction
Frank and John are pushing the
piano on its wheels to move it.

They each provide an “applied
force” in the same direction.
Meanwhile, gravitational force
and frictional forces are
working against them.

Since Frank and John’s
combined forces are greater,
then the piano moves!

The net force is “unbalanced”
Unbalanced Forces-Opposite
directions

When two forces act in
opposite directions, they
combine by subtraction,

If one force is greater
than the other force, the
object will move in the
direction of the greater
force (tug-of-war).
This force is
unbalanced
because there is a
change in motion
Unbalanced Forces



In this game of tug of war,
the red dog is losing
ground.
The applied force of the
red dog is not as great as
the applied force of the
gray dog.
Net force is unbalanced,
and in the direction of the
gray dog.
Types of Forces
Contact forces are types of
forces in which the two
interacting objects are
physically contacting each
other.
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.







Applied Force
Frictional Force
Tensional Force
Normal Force
Air Resistance Force (which is
a type of frictional force)
Spring Force



Gravitational Force
Electrical Force
Magnetic Force
Centripetal Force
APPLIED FORCE – PUSH
OR PULL
FRICTIONAL FORCE –
OF THE MOTION
GOES AGAINST THE DIRECTION
TENSIONAL FORCE –WHEN THINGS ARE PULLING
OPPOSITE DIRECTION
IN
NORMAL FORCE – OPPOSES THE GROUND
AIR
RESISTANCE FORCE
FORCE
– TYPE OF
FRICTIONAL
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 a spring-loaded
cannon. 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
Label the forces
involved in Tom
Brady’s winning
Touchdown
throw!
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! One source even
called it “the ‘evil’ of all motion” (Jackson).
There are 4 types of friction: static, sliding, rolling
and fluid. Each of these have their own unique
application and advantages. There are pro’s and
cons to friction as well as ways to decrease
friction. Understanding these things will help us
use friction to our advantage as well as lessen it
when it becomes a frustration in daily life.
WARM UP

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
surfaces are made.
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 HELPS slow down your car
 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.

WARM UP

Give an example of how
friction can be harmful
when running a car. Then
give and example of how
it could be helpful. Try
to identify the type of
friction involved in your
example.
Friction Assignment:

You will create a poster advertising a type of
friction. What your poster must include:
An illustration with the direction of forces and motion
labeled. Make sure the size of your arrows match the
size of the force.
 Include in you poster the definition of friction and the
definition of your chosen type of friction.
 Include a statement explaining how friction is playing
a roll in your illustration.
 Be neat, creative and add some color to your poster!


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?
WARM UP

Significantly decreased gravity gives astronauts
the sensation of being weightless and forces
astronauts to make many adjustments in their
activities. Write a paragraph explaining what you
would like and dislike about living with reduced
gravity.
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
Free Fall
Terminal Velocity
Air
Resistance
Gravity
gravity
Weight
Weight is
 a measure of the force
of gravity on an object
 Metric or SI unit is
the Newton (N)
•Your weight varies
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.
BALL DROP INVESTIGATION
You have been given two balls, predict which ball
will hit the ground first if dropped from the same
height.
 Now, with your neighbor, try it!
 Were the results what you expected? Why or why
not?
 Now, predict what would happen is you dropped a
piece of paper and the golf ball.
 Now try it! – what happened? Were the results
what you expected? Why or why not?
 How could you get the paper to drop at the same
rate as the golf ball?

IF I
DROP 3 BALLS OF SIMILAR SIZE AT THE SAME
TIME, WHICH WILL HIT THE GROUND FIRST?

They will hit at the same time, assuming there is
no air resistance!

Why is this??
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

CALCULATE YOUR
WEIGHT
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
WARM UP 11/27

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

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.
What must you know in order to calculate the
gravitational force between two objects?
Free Fall
Terminal Velocity
Air
Resistance
Gravity
gravity
PROJECTILE MOTION
1.
Throw a ball. What happens to the ball during
flight?
2.
Now draw the path of the ball.
3.
Consider all forces and motions acting on the
ball, try to draw these!
Projectile Motion:
Horizontal motion (inertia)
Gravity
Actual path of
baseball
DRAWING FORCE DIAGRAMS

Use your textbook pages 154-156 to draw a
diagram of projectile and orbital motion. Be sure
to include and label all motions/forces.
Orbital Motion:
Forward motion (inertia) of the
space shuttle
Gravity
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.

ORBITING OBJECTS QUESTIONS TO CONSIDER

Answer the following below your diagram:






What force put the stopper into motion?
What force is keeping the stopper in
motion?
Why is the stopper moving in a circle?
If you let go of the string, where would
the stopper go?
What force is working against the motion
of the stopper?
How is this similar and different from the
orbit of the moon around the earth?
WARM UP: 11/29

You’ve all heard the myth about the penny
dropped from the empire state building. It says
that if you were hit by this penny, you would be
seriously injured. Based on what we learned
about terminal velocity, do you think this myth is
true? Why or why not?
PROJECTILE
MOTION INVESTIGATION
You have been given 2 types of balls, try tossing
each into the bowl.
 Draw a diagram of the path of each ball as it
leaves your hand and enters the bowl.
 Label the forces involved with names and arrows,
paying careful attention to the size of your
arrows.
 Which object was more difficult to throw into the
bowl? Why?

WARM UP
If Wile E. Coyote and a boulder fall off a cliff at the
same time, which do you think will hit the ground
first?
WARM UP

You throw a ball up into the air. It reaches a
maximum height and then begins to fall back
towards the ground. How fast is it traveling after
it has been falling for 2.5 seconds?
V = gt
 V = 9.8 * 2.5
 V = 24.5 m/s downward

TOTD – 11/30/12

Suppose your friend asks you to tell them about
projectile motion. You start to explain, but she
interrupts. “Wait,” she says, “You’re using a lot of
words I don’t understand. Can you explain in
English?” Knowing that a picture is worth a
thousand words, draw a picture of a projectile
path and label all the key terms, including any
necessary explanations.