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Chapter 9: The Laws of Motion
Ms. Johnson
Foundations
Objectives
• A force is a push or pull. A force can cause an object to
be put in motion, stop motion, change its speed of
motion, or change its direction. Objects at rest are
affected by balanced forces, and objects in motion are
affected by unbalanced forces.
• Mass the measure of the amount of substance.
• Acceleration is the rate of change of velocity.
• Newton’s three laws of motion govern the motion of all
objects.
i.
Newton’s first law states that an object at rest will
remain at rest and an object in motion will remain in
motion unless another force affects it.
ii. Newton’s second law states that acceleration of an object
depends directly on the force applied and inversely on its
mass.
iii. Newton’s third law states that every action has an equal
but opposite reaction.
Vocabulary
• Force
• Mass
• Acceleration
• Newton’s First Law
• Newton’s Second Law
• Newton’s Third Law
Chapter 9: The Laws of Motion
Force: a push or pull
Examples of pushing forces:
Examples of pulling forces:
• Hitting a baseball
• Closing a refrigerator
door
• Wind knocking down a
tree
• Moving a shopping cart
• Hammering a nail
•
•
•
•
•
Pulling a tug-of-war rope
Opening a closet door
Lifting a shovel full of dirt
Climbing a rope
Gravity pulling an apple
to the ground
Force
• A force can also stop motion, change the speed
of motion, and change the direction of motion of
an object.
▫ Stopping motion: glove catching a baseball
▫ Slowing motion: car going from flat road to uphill
▫ Changing direction: tennis racket striking a tennis
ball
Force
• When forces acting on an object are balanced,
the object is at rest.
▫ Ex. if you place one book on a table, the forces are
equal and the table can support the book.
▫ Ex. but if you place a pile of books on a table and
the upward force of the table cannot hold them,
the forces are unbalanced and the table can break.
Forces
Balanced Forces
Unbalanced Forces
• Car parked in driveway
• Apple hanging on a tree
• Roller coaster on top of a
ride
• Car pulling out of driveway
• Apple falling to the ground
• Roller coaster coming down
Mass
• Mass: the measure of the amount of substance
▫ Mass remains constant and does not change
anywhere in the universe
• The greater the mass, the more force necessary
to move it.
Acceleration
• Acceleration: the rate of change of velocity
▫ Ex. A car traveling 30 kph has no acceleration
▫ If the car increases its velocity to 50 kph, it
accelerates.
▫ This means it takes additional force to cause the
car to accelerate
Chapter 9: The Laws of Motion
Newton’s Laws of Motion
• Sir Isaac Newton formulated 3 laws of motion.
▫ These laws explain how forces affect the state of
motion of all objects.
• Even presently, these laws remain the basis for
understanding motion.
First Law of Motion
• 1st Law: an object at rest will remain at rest and an
object in motion will remain in motion until an
outside force acts on the object.
▫ An object at rest will not move unless a force acts on
it.
 Ex. A bicycle will remain leaning against a tree unless
something/someone moves it.
▫ Any moving object will continue to move unless a
force acts on it to slow it or change its direction.
 Ex. A ball thrown will continue unless a force acts on itair friction slows the ball, and gravity changes its
direction pulling it down.
First Law of Motion
• Inertia: the tendency of an object to resist any change in
motion.
▫ The more massive an object, the more inertia it has (i.e.
the more it resists a change in motion).
• Ex. when riding in a moving car that stops suddenly, you
feel your body move forward.
▫ Your body is resisting the stopping action.
▫ You feel pushed forward.
Second Law of Motion
• 2nd Law: the acceleration of an object depends
directly on the force applied and inversely on its
mass.
𝑎𝑐𝑐𝑒𝑙𝑒𝑟𝑎𝑡𝑖𝑜𝑛 =
𝐹𝑜𝑟𝑐𝑒
𝑚𝑎𝑠𝑠
or
𝑎=
𝐹
𝑚
Second Law of Motion
• An increase in a force on a
given mass will increase
acceleration.
• An object with a large mass
will not be moved easily by a
small force, where a larger
▫ If an equal force is applied to two
force would move it more
objects of different mass, the
easily.
object with less mass will have a
greater acceleration.
▫ Ex. it is easier for an adult to
push a child on a swing than it
is for a child to push an adult.
▫ The child has to put in much
more force.
Third Law of Motion
• 3rd Law: for every action there is an equal and
opposite reaction.
▫ Ex. blowing up a balloon and releasing it.
 When air is released from the balloon, the balloon
moves in the opposite direction of the air.
Chapter 9: The Laws of Motion