Laws of Motion Notes - Independent School District 196

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Transcript Laws of Motion Notes - Independent School District 196

Newton’s Laws of
Motion
Forces and Motion
What are forces?
• A force is a push or pull on an
object.
• Sometimes it is easy to see when
a force is being applied, however,
sometimes it is not as obvious.
• What does this mean?
Changing Motion
• A force can cause an object to
change motion.
Net Force
• Forces are often measured in units
called NEWTONS (N)
• This symbol (Σ) is the Greek letter
sigma. We use it for shorthand to
represent “SUM” which of course
means total.
• So ΣF means “sum of forces”
Net Force
• The sum total of all the forces on an object
is the net force.
• Forces applied in the same direction add
together
• Forces applied in opposite directions
subtract from one another
Balanced Forces
• Balanced Force = forces that
are equal in size but opposite in
direction.
• No change in motion or velocity
• Net force is always ZERO
Unbalanced Forces
• Unbalanced Forces = forces
applied to an object in varying
sizes and directions.
• Cause a change in motion or
velocity
• Net force is NOT zero
What is gravity?
• Gravity is the
attractive force
between two
objects.
Law of Gravity
• All objects that have mass have gravity.
• The pull of gravity between any 2 objects
depends upon the mass of the 2 objects
and the distance between them.
Universal Law of
Gravitation
• F = Force of
gravity
• G = Gravity
constant (6.67 x
10-11 N(m/kg)2
• D = distance
between objects
• m1 and m2 =
mass of the
objects
Relationships
• As the distance between two
objects increases the force of
gravity decreases dramatically.
• No matter how far apart two
objects get the force of gravity
never goes away.
Lets Practice!
• Earth has a mass of 5.97x1024
kg. The Earth and moon are
separated by 384,400 km. The
moon has a mass of 7.34x1022
kg. What is the gravitational
force?
Section Check
Question 1
A force is a __________.
Answer
A force is a push or pull. Forces, such as the
force of the atmosphere against a person’s
body, are not always noticeable.
Section Check
Question 2
When are forces on an object balanced?
Answer
When forces are equal in size and opposite in
direction, they are balanced forces, and the
net force is zero.
Section Check
Question 3
Gravity is an attractive force between any
two objects and depends on __________.
Answer
Gravity is an attractive force between any two
objects and depends on the masses of the
objects and the distance between them.
Weight vs. Mass
Weight
• Weight is a
measurement of
gravitational pull.
• Weight changes
when the force of
gravity changes
Mass
• Mass depends upon
amount of matter in
something, not upon
gravity.
• Mass doesn’t change
when the force of
gravity changes.
However on the Earth, they are basically the same.
Mass vs. Weight
on the
Moon
•
•
•
•
•
•
on the
Moon
5 school books
Backpack (full)
Computer monitor
Globe
Person
One of choice
Newton’s Laws of
Motion
1st, 2nd, and 3rd Law
Newton’s 1st Law
INERTIA
Give me the law!
• Newton’s First Law of Motion
says that an object moving at a
constant velocity will continue
moving at that velocity unless
an unbalanced net force acts on
the object.
• The law also says that an object
at rest will stay at rest unless
acted on by an unbalanced net
force.
Newton’s 1st
Law of Motion
• This law is sometimes
called the law of Inertia
• Inertia is the property of
an object to resist any
change in motion.
• If an object is moving it
will continue to move at
the same speed and in
the same direction
•
deck of cards in space
Inertia and Mass
• If an object is at rest it will stay
at rest. Its velocity is zero
unless a force makes it move.
• The inertia of an object depends
on its mass. The more mass an
object has the more it resists a
change in motion = greater
inertia.
GALILEO, SPEED, AND
VELOCITY
•Slow and fast not good enough.
Speed = Distance travelled divided by Time travelling
S=D/T
Speed units: MPH, m/s, ft/sec, etc
Velocity =how fast and in what direction
Constant speed =speed not changing
Constant velocity =speed & direction not changing
Speed &
Velocity
• Can these cars have a constant
speed?
• Yes
• Can these cars have a constant
velocity?
• No
Newton’s 2nd Law:
RELATIONSHIP BETWEEN FORCE,
MASS, AND ACCELERATION
Newton’s 2nd Law of
Motion
• This law says that the
more force applied to an
object, the more it will be
accelerated, and thus the
more it will change its
motion.
• Acceleration = Motion that
is not constant .
• Example #1. The harder you
throw a baseball, the more
distance it will travel.(other
variables not considered here)
Force and Acceleration
• What’s different about
throwing a ball
horizontally as hard as
you can and tossing it
gently?
• When you throw hard,
you exert a much
greater force on the
ball.
Getty Images
Force and Acceleration
• The hard-thrown ball has
a greater change in
velocity, and the change
occurs over a shorter
period of time.
Getty Images
Force and Acceleration
• Recall that acceleration is the change in
velocity divided by the time it takes for the
change to occur.
• So, a hard-thrown ball has a greater
acceleration than a gently thrown ball.
Mass and Acceleration
• If you throw a softball
and a baseball as hard as
you can, why don’t they
have the same speed?
• The difference is due to
their masses.
Getty Images/Lars A. Niki
Mass and Acceleration
• If it takes the same amount of time to throw
both balls, the softball would have less
acceleration.
• The acceleration of an object depends on its
mass as well as the force exerted on it.
• Force, mass, and acceleration are
related.
Newton’s Second Law
• Newton’s second law of motion states that
the acceleration of an object is in the same
direction as the net force on the object, and
that the acceleration can be calculated from
the following equation:
Calculating Net Force with the
Second Law
• Newton’s second law also can be used to
calculate the net force if mass and
acceleration are known.
• To do this, the equation for Newton’s second
law must be solved for the net force, F.
Calculating Net Force with the
Second Law
• To solve for the net force, multiply both
sides of the equation by the mass:
• The mass, m, on the left side cancels,
giving the equation:
Let’s see what you got!
• What is the
acceleration of
“The Bat” if it
has a mass of
1,200kg and is
propelled by a
force of 24,000
N?
Just one more!
• What is the net
force Aaron
Rodgers applies
to a football
when the mass
of the ball is
0.4kg and is
accelerating at
20 m/s2
Newton’s 3rd Law
ACTION/ REACTION
Newton’s 3rd Law of
Motion
• This law states
that for every
action, there is
a opposite and
equal reaction.
• These are called
action/reaction
forces.
• Example. #1
• Heading a soccer ball
• Answer:
• Action= ball moves
upward
• Reaction= head and neck
compress.
Newton’s Third Law
• But, the action force doesn’t cause the
reaction force. They occur at the same
time.
• When you jump on a trampoline, for
example, you exert a downward force on
the trampoline.
• Simultaneously, the trampoline exerts an
equal force upward, sending you high into
the air.
Action and Reaction Forces
Don’t Cancel
• According to the third law of motion, action
and reaction forces act on different objects.
• Thus, even though the forces are equal, they
are not balanced because they act on
different objects.
Newton’s 3rd Law of
Motion
• 2) Describe how
Newton’s 2nd
Law works on
the release of a
balloon filled
with air?
• What are the
action/reaction
forces?
Newton’s 3rd
Law of Motion
• Example#3:
On ice skates, you push
against the boards, you
move backwards. What do
the boards do?
Answer:
The boards go backwards
What are the action/reaction
forces?
Section Check
Section Check
Question 1
Inertia is __________.
A. the tendency of an object to resist any
change in its motion
B. the tendency of an object to have a positive
acceleration
C. The tendency of an object to have a net
force of zero.
D. The tendency of an object to change in
speed or direction.
Section Check
Answer
Inertia is the tendency of an object to resist
any change in its motion. An unbalanced force
must act upon the object in order for its motion
to change.
Section Check
Section Check
Question 2
Newton’s second law of motion states that
_________ of an object is in the same
direction as the net force on the object.
A.
B.
C.
D.
acceleration
momentum
speed
velocity
Section Check
Answer
The answer is A. Acceleration can be
calculated by dividing the net force in newtons
by the mass in kilograms.
Section Check
Section Check
Question 3
According to Newton’s third law of motion,
what happens when one object exerts a force
on a second object?
Answer
According to Newton’s law, the second object
exerts a force on the first that is equal in
strength and opposite in direction.