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NEWTON’S 3 LAWS
NEWTON’S 1ST LAW OF MOTION
The law of inertia
FORCE
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A Force is a push or pull that one body exerts on
another.
Net force: The combination of 2 or more forces
acting on an object at the same time.
The applied force from the kick will change the
motion of this soccer ball.
FORCE
Balanced Forces


Forces that are equal
in size and opposite in
direction.
NET FORCE = 0
Unbalanced Forces

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A force that is not
balanced by another
force.
NET FORCE ≠ 0
UNBALANCED FORCES
Unequal forces in
opposite direction

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Example: 2 students
are pushing a box in
opposite directions,
but one is using more
force.
The box will move in
the direction of the
larger force.
Force exerted in the
same direction


Example: 2 students
are pushing a box in
the same direction.
The forces are added
together because they
are exerted on the box
in the same direction.
ST
INERTIA-1
LAW
THE TENDENCY OF AN OBJECT TO RESIST ANY CHANGE IN MOTION
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If an object is moving, it will keep moving at the
same speed in the same direction unless an
unbalanced force acts on it.
The skater will keep sliding on the ice until an
unbalanced force acts on her.
If an object is at rest, it tends to remain at rest…
DOES MASS MATTER?
YES


The greater the mass of an object the greater
inertia it has.
Which ball requires a greater force to change its
motion?
WHAT HAPPENS IN A CAR CRASH?
½ the people who die in car crashes
would survive if they wore a
seatbelt.

If the car crashes
going at a speed of 50
km/h, and the
passenger is not
wearing a seatbelt,
how fast will they hit
the dashboard?
NEWTON’S 2ND LAW
Force, mass and Acceleration
LET’S REVIEW…

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Newton’s 1st law says that motion of an object
only changes when an unbalanced force acts on
it….
How does force cause motion to change?
THE GREATER THE FORCE, THE GREATER
THE ACCELERATION
FORCE AND MASS
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So we threw a baseball…the greater the force
we throw it with, the greater the acceleration.
What if we throw a baseball and a tennis ball
with the same force?
The baseball will go faster because it has more
mass.
TO SUM UP NEWTON’S 2ND LAW…

Force, mass and acceleration are all related!
Acceleration = force/mass
 a= F/m

Units mass uses kg, acceleration uses m/s², so
force would be (kg x m/s²).
 Too confusing, we call it the newton (N).

PRACTICE PROBLEM

You are pushing your friend on a sled. You push
with a force of 40 N. Your friend and the sled
together have a mass of 80 kg. Ignoring friction,
what is the acceleration of your friend on the
sled?
ANSWER…
a= F/m
 Acceleration = ?
 Force = 40 N
 Mass = 80 kg

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a= 40 N/80 kg
a = .5m/s²
Could we find force if we were just given mass
and acceleration? What would the equation look
like?
http://www.physicsclassroom.com/Class/newtlaws/u2l3a.cfm
REAL LIFE APPLICATIONS OF THE 2ND LAW


Dropping anything from a height above the surface of the earth.
The FORCE of gravity acting on the MASS you drop produces
an ACCELERATION.
A rocket launch. The FORCE of the rockets acts on the MASS of
the rocket and produces and ACCELERATION.
Anything that is moving (cars, baseballs, bugs)
has it's motion explained by the NET FORCE
acting on a MASS.

FRICTION
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If you push a skateboard, why does it eventually stop?
FRICTION.
Friction is the force that opposes motion between
surfaces that are touching each other.
3 types of friction
 (1) static friction (2) sliding friction (3) rolling friction

STATIC FRICTION

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The friction between 2 surfaces that are not
moving past each other.
Example: You are trying to move a box that is
extremely heavy. You are pushing but the box is
not moving. Since the acceleration is 0, the force
you exert is being cancelled out by another
force…static friction!
SLIDING FRICTION


The force that opposes the motion of 2 surfaces
sliding past each other.
Example: You push a skateboard down the street
and it eventually stops. The force that causes the
acceleration to become 0 is sliding friction.
ROLLING FRICTION


The friction between a rolling object and the
surface it rolls on.
Example: When you ride your bike there is
friction between the tires and the ground. Rolling
friction is usually much less than static or
sliding.
HOW DOES FRICTION AFFECT A SPORT YOU PLAY?

Talk with your partner and write down 5
examples of how friction is involved in a sport
you play, or a hobby you practice.
AIR RESISTANCE
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Air resistance affects anything that moves in Earth’s atmosphere.
Air resistance is a force that acts in the opposite direction of the
object’s motion.
The amount of air resistance depends on the speed, size, and
shape of the object. (Cross-sectional area important!)
If there were no air resistance an apple and a feather would fall
at the same rate!
http://www.youtube.com/watch?v=WyBYVQzvGdI&feature=related
LET’S CRUMPLE SOME PAPER…
Grab a partner and sit at a lab table.
 Bring 4 pieces of paper with you that you don’t
mind crumpling.
 Step 1. Drop a non-crumpled paper from the
table and time how long it takes to reach the
ground. Record.
 Step 2. Repeat with a loosely crumpled paper.
 Step 3. Repeat with a tightly crumpled paper.
 Step 4. Repeat with a piece of paper designed by
you to have the LEAST air resistance.

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Contest for who designs a paper with the least
air resistance!
PROBLEMS
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1. 2 people push a box in the same direction, one
person uses twice as much force as the other.
Represent this with force vectors.
2. 2 people push a box in opposite directions. They
are of equal strength. Draw this with force vectors,
using the same scale as in #1.
3. If a box is pushed with a force of 15 N and it has a
mass of 3 kg, how fast is it accelerating?
4.How much force must you apply to move a car of
4,000 kg with an acceleration of 3 m/s2?
THE LAW OF
GRAVITATION
GRAVITY
Anything that has mass is attracted by the force of
gravity.
 Any 2 masses exert an attractive force on each
other.
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Mass 1
Mass 2
You are attracting your desk, the person next to
you, your pencil…
You can only feel the gravitation attraction to the
earth because its mass is so large! (You are
attracting your desk right now, but its mass is too
small and the force is too weak to feel it)
GRAVITY DEPENDS ON MASS AND DISTANCE!
If the mass of either object
increases, the gravitational force
between them increases.
If the objects are closer
together, the gravitational
force between them increases.
GRAVITY IS LONG RANGE
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Even though
stars are SO far
away from each
other there is
still
gravitational
attraction
between them.
Gravity is what
gives our
universe its
shape!
There is gravitational force between ALL matter in the universe
ACCELERATION DUE TO GRAVITY
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The gravity on earth causes all falling objects to have
an acceleration of 9.8 m/s².
Weight: The gravitational force exerted
on an object.
F = m x a (Newton’s second law)
 Gravitational force = m x acceleration due to gravity
 Weight = m x 9.8 m/s²
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http://www.physicsclassroom.com/Class/newtlaws/u2l3c.cfm
MASS ≠ WEIGHT !!
Weight is a FORCE
 Mass measures the amount of MATTER an object has
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Let’s practice.
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http://www.physicsclassroom.com/Class/newtlaws/u2l2b.cfm#friction
FREE FALL AND AIR RESISTANCE
But why do all objects free fall at the same rate of
acceleration regardless of their mass?
 Is it because they all weigh the same?
 they all have the same gravity?
 the air resistance is the same for each?
 Why?

FREE FALL
HTTP://WWW.PHYSICSCLASSROOM.COM/MMEDIA/NEWTLAWS/EFFF.CFM

When the only force acting upon the object is the
force of gravity.
PROJECTILE MOTION
PROJECTILE MOTION
When you toss a ball it tends to curve downward. This is
due to Earth’s gravitational pull.
 Anything thrown or shot through the air = projectile
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Projectiles have horizontal and vertical velocities
that are independent of each other.
CENTRIPETAL FORCE
CENTRIPETAL FORCE
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Motion in a curved path is accelerated motion
(because of the constant direction change)and
requires a force toward the center of the path.
Remember, any object in acceleration must be acted
upon by an unbalanced force.
The net force during centripetal force is directed
inward.
 An inward net force is required to make a turn in a
circle
 In the absence of any net force, an object in motion
(such as a passenger in a car) continues in motion in
a straight line at constant speed.

NEWTON’S THIRD LAW
For every action, there is an equal and opposite reaction.