Newton's Laws of Motion

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Newton's Laws of Motion
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Essential Question

• How can I explain the causes of motion on an
  object?
• Learning Objectives Laws of Motion, Friction,
  Force

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Background

• Sir Isaac Newton (1643-1727) an English scientist
  and mathematician famous for his discovery of the
  law of gravity also discovered the three laws of
  motion. He published them in his book
  Philosophiae Naturalis Principia Mathematica
  (mathematic principles of natural philosophy) in
  1687. Today these laws are known as Newtons
  Laws of Motion and describe the motion of all
  objects on the scale we experience in our
  everyday lives.

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Newtons Laws of Motion

• 1st Law An object at rest will stay at rest,
  and an object in motion will stay in motion at
  constant velocity, unless acted upon by an
  unbalanced force.
• 2nd Law Force equals mass times acceleration.
• 3rd Law For every action there is an equal and
  opposite reaction.

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Newton's Laws of Motion

• First Law An object at rest stays at rest or an
  object in motion, stays in motion (in the same
  direction/at the same speed) unless acted upon by
  an unbalanced force
• Also called the law of inertia

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• Inertia
• A property of matter
• The tendency of an object to resist any change in
  its motion
• The greater the mass the greater the inertia
• The greater the speed the greater the inertia

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Examples of Newtons 1st Law
a) car suddenly stops and you strain against the
seat belt b) when riding a horse, the horse
suddenly stops and you fly over its head c) the
magician pulls the tablecloth out from under a
table full of dishes d) the difficulty of
pushing a dead car e) lawn bowling on a cut and
rolled lawn verses an uncut lawn appear f) car
turns left and you to slide to the right
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Examples of Newtons 1st Law
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1st Law

• Once airborne, unless acted on by an unbalanced
  force (gravity and air fluid friction), it
  would never stop!

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1st Law

• Unless acted upon by an unbalanced force, this
  golf ball would sit on the tee forever.

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Newtons First Law

• Newtons First Law of Motion
• An object at rest will remain at rest and an
  object in motion will continue moving at a
  constant velocity unless acted upon by a net
  force.

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Newtons First Law

• Newtons First Law of Motion
• Law of Inertia
• Inertia
• tendency of an object to resist any change in its
  motion
• increases as mass increases

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Some Examples from Real Life
A soccer ball is sitting at rest. It takes an
unbalanced force of a kick to change its motion.

• Two teams are playing tug of war. They are both
  exerting equal force on the rope in opposite
  directions. This balanced force results in no
  change of motion.

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More Examples from Real Life
A powerful locomotive begins to pull a long line
of boxcars that were sitting at rest. Since the
boxcars are so massive, they have a great deal of
inertia and it takes a large force to change
their motion. Once they are moving, it takes a
large force to stop them.
On your way to school, a bug flies into your
windshield. Since the bug is so small, it has
very little inertia and exerts a very small force
on your car (so small that you dont even feel
it).
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If objects in motion tend to stay in motion, why
dont moving objects keep moving forever?
Things dont keep moving forever because theres
almost always an unbalanced force acting upon it.
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.
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• Why then, do we observe every day objects in
  motion slowing down and becoming motionless
  seemingly without an outside force?
• Its a force we sometimes cannot see friction.

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What is this unbalanced force that acts on an
object in motion?
Friction!

• 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

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Newtons First Law

• An object at rest tends to stay at rest and an
  object in motion tends to stay in motion unless
  acted upon by an unbalanced force.

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What does this mean? Review

• Basically, an object will keep doing what it was
  doing unless acted on by an unbalanced force.
• If the object was sitting still, it will remain
  stationary. If it was moving at a constant
  velocity, it will keep moving.
• It takes force to change the motion of an object.

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Newtonss 1st Law and You
Dont let this be you. Wear seat belts. Because
of inertia, objects (including you) resist
changes in their motion. When the car going 80
km/hour is stopped by the brick wall, your body
keeps moving at 80 m/hour.
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2nd Law

• The net force of an object is equal to the
  product of its mass and acceleration, or Fma.

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Newton's Laws of Motion

• Second law The greater the force applied to an
  object, the more the object will accelerate. It
  takes more force to accelerate an object with a
  lot of mass than to accelerate something with
  very little mass.

The player in black had more acceleration thus he
hit with a greater amount of force
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2nd Law
F m x a
What is Force? It is any push or pull on an
object!!
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Newton's Laws of Motion

• Second law
• The greater the force, the greater the
  acceleration
• The greater the mass, the greater the force
  needed for the same acceleration
• Calculated by F ma
• (F force, m mass, a acceleration)

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Examples of Newtons 2nd Law
a) hitting a baseball, the harder the hit, the
faster the ball goes b) accelerating or
decelerating a car c) The positioning of
football players - massive players on the line
with lighter (faster to accelerate) players in
the backfield d) a loaded versus an unloaded
truck
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Examples of Newtons 2nd Law
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Newtons Second Law

• Newtons Second Law of Motion
• The acceleration of an object is directly
  proportional to the net force acting on it and
  inversely proportional to its mass.

F ma
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Newtons Second Law

• Force equals mass times acceleration.
• F ma

Acceleration a measurement of how quickly an
object is changing speed.
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2nd Law

• When mass is in kilograms and acceleration is in
  m/s/s, the unit of force is in newtons (N).
• One newton is equal to the force required to
  accelerate one kilogram of mass at one
  meter/second/second.

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2nd Law (F m x a)

• How much force is needed to accelerate a 1400
  kilogram car 2 meters per second/per second?
• Write the formula
• F m x a
• Fill in given numbers and units
• F 1400 kg x 2 meters per second/second
• Solve for the unknown
• 2800 kg-meters/second/second or 2800 N

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Newtons 2nd Law proves that different masses
accelerate to the earth at the same rate, but
with different forces.

• We know that objects with different masses
  accelerate to the ground at the same rate.
• However, because of the 2nd Law we know that they
  dont hit the ground with the same force.

F ma 98 N 10 kg x 9.8 m/s/s
F ma 9.8 N 1 kg x 9.8 m/s/s
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ConceptTest

• Is the following statement true or false?
• An astronaut has less mass on the moon since the
  moon exerts a weaker gravitational force.

• False! Mass does not depend on gravity, weight
  does. The astronaut has less weight on the moon.

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Check Your Understanding

• 1. What acceleration will result when a 12 N net
  force applied to a 3 kg object? A 6 kg object?
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• 2. A net force of 16 N causes a mass to
  accelerate at a rate of 5 m/s2. Determine the
  mass.
• 3. How much force is needed to accelerate a 66 kg
  skier 1 m/sec/sec?
• 4. What is the force on a 1000 kg elevator that
  is falling freely at 9.8 m/sec/sec?

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Newton's Laws of Motion

• third law For every action force, there is an
  equal and opposite reaction force. (Forces are
  always paired)

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3rd Law

• According to Newton, whenever objects A and B
  interact with each other, they exert forces upon
  each other. When you sit in your chair, your body
  exerts a downward force on the chair and the
  chair exerts an upward force on your body.

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Newtons 3rd Law in Nature

• Consider the propulsion of a fish through the
  water. A fish uses its fins to push water
  backwards. 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
  the direction of the force on the fish (forwards).

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3rd Law
Flying gracefully through the air, birds depend
on Newtons third law of motion. As the birds
push down on the air with their wings, the air
pushes their wings up and gives them lift.
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Examples of Newtons 3rd Law

• rockets leaving earth
• guns being fired
• c) two cars hit head on
• d) astronauts in space
• e) pool or billiards
• f) jumping out of a boat onto
• the dock
• g) sprinklers rotating

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Examples of Newtons 3rd Law
Newtons third law "For every action, there is
an equal and opposite reaction." When you fire a
gun you feel the recoil. Some of the funniest
things in cartoons follow physics that have been
exaggerated or just plain ignored. Wyle Coyote
hangs suspended in space over that canyon for a
lot longer than an object would in reality, but
it is the anticipation of the drop and Wyle's
facial recognition of the upcoming pain that is
so classically cartooney. So some laws are
stretched for comical effect.
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Other examples of Newtons Third Law

• The baseball forces the bat to the left (an
  action) the bat forces the ball to the right
  (the reaction).

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3rd Law

• Consider the motion of a car on the way to
  school. A car is equipped with wheels which spin
  backwards. As the wheels spin backwards, they
  grip the road and push the road backwards.

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Examples of Newtons 3rd Law
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Newtons Third Law

• Newtons Third Law of Motion
• When one object exerts a force on a second
  object, the second object exerts an equal but
  opposite force on the first.

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Newtons Third Law

• Problem

• How can a horse pull a cart if the cart is
  pulling back on the horse with an equal but
  opposite force?

• Arent these balanced forces resulting in no
  acceleration?

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Newtons Third Law

• Explanation

• forces are equal and opposite but act on
  different objects
• they are not balanced forces
• the movement of the horse depends on the forces
  acting on the horse

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Newtons Third Law

• Action-Reaction Pairs

• The hammer exerts a force on the nail to the
  right.
• The nail exerts an equal but opposite force on
  the hammer to the left.

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Newtons Third Law

• Action-Reaction Pairs

• The rocket exerts a downward force on the exhaust
  gases.
• The gases exert an equal but opposite upward
  force on the rocket.

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Newtons Third Law

• For every action there is an equal and opposite
  reaction.

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What does this mean?
For every force acting on an object, there is an
equal force acting in the opposite direction.
Right now, gravity is pulling you down in your
seat, but Newtons Third Law says your seat is
pushing up against you with equal force. This is
why you are not moving. There is a balanced
force acting on you gravity pulling down, your
seat pushing up.
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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).
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Review
Newtons 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.
Newtons Second Law
Force equals mass times acceleration (F ma).
Newtons Third Law
For every action there is an equal and opposite
reaction.
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Newtons Laws of Motion

• An object in motion tends to stay in motion and
  an object at rest tends to stay at rest unless
  acted upon by an unbalanced force.
• Force equals mass times acceleration (F ma).
• For every action there is an equal and opposite
  reaction.

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Which of Newtons Laws of Motion apply to these
situations?

• An object rests in your hand
• A ball is tossed into the air
• A car windshield hits a bug
• A person sits on a table
• A person jumps up from the floor
• A baseball bat hits a baseball
• A truck and car hit head on
• Any others?

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