NEWTON’S LAWS OF MOTION
st
1 Law
Inertia
• An object at rest will stay at rest.
st
1 Law
Inertia
• An object in motion will stay in motion
st
1 Law
Inertia
An object will stay in its current condition
until acted upon by another force.
Balanced Force
Equal forces in opposite
directions produce no motion
Unbalanced Forces
Unequal opposing forces
produce an unbalanced force
causing motion
If objects in motion tend to stay in motion,
why don’t moving objects keep moving
forever?
Things don’t keep moving forever because
there’s almost always an unbalanced force
acting upon them.
A book sliding across a table slows
down and stops because of the force
of friction.
If you throw a ball upwards it will
eventually slow down and fall
because of the force of gravity.
Newton’s First Law – Law of Inertia
Mass is the measure of the amount of
matter in an object.
Inertia is a property of an object that
describes how much it will resist change
to the motion of the object
More mass = more Inertia
Newton’s First Law – Law of Inertia
Unless acted
upon by an
unbalanced
force, this
soccer ball
would sit on
the ground
forever.
What is this unbalanced force that acts on an
object in motion?
There are four main types of friction:
– Sliding friction: ice skating
– Rolling friction: bowling
– Fluid friction (air or liquid): air or water resistance
– Static friction: initial friction when moving an object
Newton’s First Law – Law of Inertia
Once airborne,
unless acted on
by an
unbalanced force
(gravity and air –
fluid friction) the
soccer ball would
never stop!
Inertia
st
How To Apply Newton’s 1 Law of
Motion
st
How To Apply Newton’s 1 Law of
Motion
Estimated 19
Miles/Gallon
HWY
Estimated 9
Miles/Gallon
City
WHY?????
nd
Newton’s 2 Law
of Motion
Once an object is moving, it will move in a
straight line until acted upon by another
force.
Force = ma
(F=ma)
Force = Mass x Acceleration (F=ma)
Newton’s Second Law
ACCELERATION of GRAVITY (Earth) = 9.8 m/s2
Weight (force) = mass x gravity (Earth)
Moon’s gravity is 1/6 of the Earth’s
If you weigh 420 Newtons on earth,
what will you weigh on the Moon?
70 Newtons
Newton’s Second Law
One rock weighs 5
Newtons. The other rock
weighs 0.5 Newtons. How
much more force will be
required to accelerate the
first rock at the same rate
as the second rock?
Ten times as much
How To Apply
Newton’s 2nd Law
of Motion
How To Apply Newton’s 2nd Law of Motion
How to apply F=ma
Before…
How to apply F=ma
After!
rd
Newton’s 3 Law of Motion
For every action there is an EQUAL and
OPPOSITE reaction.
rd
Newton’s 3 Law of Motion
Book to
earth
Table to
book
Action and Reaction on Different Masses
Action: earth pulls on you
Reaction: you pull on earth
Reaction: gases push on rocket
Action: rocket pushes on gases
Think about it . . .
What happens if you are standing on a
skateboard or a slippery floor and push
against a wall? You slide in the opposite
direction (away from the wall), because you
pushed on the wall but the wall pushed back
on you with equal and opposite force.
Why does it hurt so much when you stub your
toe? When your toe exerts a force on a rock, the
rock exerts an equal force back on your toe.
The harder you hit your toe against it, the more
force the rock exerts back on your toe (and the
more your toe hurts).
rd
Newton’s 3 Law of Motion
A bug with a mass of
5 grams flies into the
windshield of a moving
1000kg bus.
Which will have the
most force?
The bug on the bus
The bus on the bug
rd
Newton’s 3 Law of Motion
Without the
acceleration, it is
impossible to
know!
Force (bug)= m x
Force (bus)=
a
mxa
How To Apply Newton’s
rd
3 Law of Motion
How To Apply Newton’s
rd
3 Law of Motion
Consider hitting a baseball with a
bat. If we call the force applied to
the ball by the bat the action force,
identify the reaction force.
(a) the force applied to the bat by the hands
(b) the force applied to the bat by the ball
(c) the force the ball carries with it in flight
(d) the centrifugal force in the swing
(b)
rd
Newton’s 3 Law of Motion
A rich merchant finds himself
stranded in the middle of a frozen
lake. Don't ask how he got there.
Think Twilight Zone... The surface
is perfectly frictionless. All he has
is his clothing and a large bag of
gold coins. How can he save
himself?
What Laws are represented?
Review
Newton’s First Law:
Objects in motion tend to stay in motion
and objects at rest tend to stay at rest
unless acted upon by an unbalanced force.
Newton’s Second Law:
Force equals mass times acceleration
(F = ma).
Newton’s Third Law:
For every action there is an equal and
opposite reaction.
1stlaw: Homer is large and has
much mass, therefore he has much
inertia. Friction and gravity oppose
his motion.
2nd law: Homer’s mass x 9.8 m/s/s
equals his weight, which is a force.
3rd law: Homer pushes against the
ground and it pushes back.
A person is traveling at 20 m/s in
a car when the car hits a tree.
The person comes to a complete
stop in 0.4 seconds. What was
the person’s acceleration?
A person is traveling at 20 m/s in
a car when the car hits a tree.
The person comes to a complete
stop in 0.4 seconds. What was
the person’s acceleration?
a = (0 m/s – 20 m/s)/0.4 s = –50 m/s2
You throw a ball to a friend. If
the ball has a mass of0.15kg and
it accelerates at 20m/s, what
force did you exert on the ball.
You throw a ball to a friend. If the
ball has a mass of0.15kg and it
accelerates at 20m/s, what force
did you exert on the ball.
F=mxa
F = 0.15 x 20 = 3N
You throw a 0.5 kg ball with a
force of 10N. What is the ball’s
acceleration?
You throw a 0.5 kg ball with a
force of 10N. What is the
ball’s acceleration?
M = 0.5
F = 10 N
Acceleration = a
A = F/m
10 N / 0.5 kg = 20m/s2
Force & Motion
Thinking Questions
Can an object’s acceleration be a
negative number? Why or why
not?
Can an object’s acceleration be a
negative number? Why or why
not?
Yes. When an object slows down and the
final speed is less than the initial speed,
the object’s acceleration will be a negative
number.
If forces occur in equal but
opposite pairs, how can anything
ever move?
If forces occur in equal but
opposite pairs, how can anything
ever move?
According to Newton’s third law, the equal
and opposite forces work on different
objects.
Read more about this here:
http://www.mansfieldct.org/schools/mms/staff
/hand/Lawshowcananythingmove.htm
Two crates, one heavy and one
light, are at rest on a waxed floor.
Which crate will need a greater
force to provide the same
acceleration? Use the concept
of inertia to explain your
answer.
Two crates, one heavy and one light,
are at rest on a waxed floor. Which
crate will need a greater force to
provide the same acceleration? Use
the concept of inertia to explain
your answer.
The heavy crate has more inertia, so it
requires more force to make it accelerate.
Explain the role of friction in
walking on a sidewalk versus
walking on a sidewalk covered
with ice.
Explain the role of friction in
walking on a sidewalk versus
walking on a sidewalk covered
with ice.
Friction between your shoe and the
sidewalk allows you to push against the
sidewalk. Ice decreases this friction.
You sit on a rock. Your weight is
a force acting on the rock. What
force acts on you?
You sit on a rock. Your weight is
a force acting on the rock. What
force acts on you?
The rock provides an upward
force that is equal to your weight
since you are not moving.
How is acceleration related to
change in speed?
How is acceleration related to
change in speed?
Acceleration is the change in velocity
divided by the time needed for the
change to occur, and velocity changes
when speed changes.
Forces & Motion
Review
Describe Acceleration
Describe Acceleration
• A change in velocity – which may be:
– A change in speed
•
•
•
•
Starting
Stopping
Speeding up
Slowing down
– A change in direction
• Acceleration is caused by unbalanced
forces
More
Describe Acceleration
Deceleration is also called negative
acceleration - it means an object is
slowing down
When acceleration is calculated, it may be a
negative number
Describe Speed
Describe Speed
• A way to describe motion
– Average speed - Rate of motion calculated by dividing
the distance traveled by the amount of time it takes to
travel that distance
– Constant speed - Speed that does not change
– Instantaneous speed - Speed of an object at any
given time
What is the formula used to
calculate speed?
What is the formula for
calculating speed?
Speed is calculated by dividing distance
by time –
Calculate This Speed
A football field is about 100 m long. If it
takes a person 20 seconds to run its
length, how fast was the football player
running?
Calculate this Speed:
A football field is about 100 m long. If it
takes a person 20 seconds to run its
length, how fast was the football player
running?
Speed = Distance ÷ Time
Speed = 100 m ÷ 20 s
Speed = 5m/s
Remember
to include
the UNITS!!
Explain Balanced Forces
Explain Balanced Forces
• When all the forces acting on an object
balance each other
• Balanced forces do not cause a change in
motion
Describe Friction
Describe Friction
• Force that resists motion between two
touching surfaces
• Acts in the opposite direction of the
object’s motion
• Produces heat
Explain Inertia
Explain Inertia
• Moving objects tend to continue moving unless
acted upon by an unbalanced force
• Objects at rest tend to stay at rest unless acted
upon by an unbalanced force
• The more mass an object has, the more inertia it
has
– More massive objects are harder to start moving and
stop moving
– Smaller objects are easier to start and stop moving
More
Explain Inertia
Newton’s First Law on Motion describes the
idea of inertia
• An object at rest or in constant motion is
acted upon by balanced forces – an
unbalanced force will change the motion
• Acceleration of an object at rest or in
constant motion is 0 m/s/s (no motion)
Explain Newton’s First Law
of Motion
Explain Newton’s First Law of
Motion
• Describes the idea of inertia
Click the link below to observe the law
http://archive.ncsa.uiuc.edu/Cyberia/VideoTestbed/Projects/NewPhysic
s/newtons_1.html
When you are finished, click the back button
on your browser to return to this tutorial
Explain Newton’s Second Law of
Motion
Explain Newton’s Second Law of
Motion
• Describes motion created by unbalanced forces
• Mass and acceleration change in opposite ways
– The more mass an object has, the more force it
take to accelerate the object, the slower it
accelerates
– The less mass an object has, the less force it
take to accelerate the object, the faster it
accelerates
More
Explain Newton’s Second Law of
Motion
Click on the link below to observe the law:
http://archive.ncsa.uiuc.edu/Cyberia/VideoTestbed/Projects/NewPhysics/n
ewtons_2.html
When you are finished, click the back button
on your browser to return to this tutorial
Explain Newton’s Third Law
of Motion
Explain Newton’s Third Law of
Motion
• Describes why forces act in pairs
• For every action there is an equal and opposite
reaction
• Action and reaction forces are equal forces
acting in opposite directions on different objects
Explain Newton’s Third Law of
Motion
Click the link below to observe the law
http://archive.ncsa.uiuc.edu/Cyberia/VideoTestbed/Projects/NewPhysics/
newtons_3.html
Distinguish Between Speed
& Velocity
Distinguish Between Speed and
Velocity
• Speed describes distance and time
• Velocity describes distance, time, and
direction
How Can Forces Affect
Objects?
How Can Forces Affect Objects?
•
•
•
•
•
•
Slow them down
Speed them up
Stop them
Start them
Change their direction
Change their shape
Click here for more review:
http://www.usoe.k12.ut.us/curr/sci
ence/sciber00/8th/forces/sciber/int
ro.htm