1. Newton’s Laws of Motion
Physical Science
9th Grade
Designed to demonstrate each law and
encourage students to see their application
2. Sir Isaac Newton
• Newton built on the
work of scientists such
as Galileo.
• Published his works
after many years in a
book entitles Principia.
• He first had to define
force and mass. He
then defined his laws
of motion.
3. Newton’s First Law of Motion
• Newton’s first law of motion states that
the motion of an object does not change
as long as the net force acting on the
object is zero.
• Thus, unless an unbalanced force acts, an
object at rest remains at rest, and an
object in motion remains in motion with
the same speed and direction.
4. Newton’s First Law of Motion
• Often called the Law of Inertia.
–Inertia is the tendency of an object to
resist a change in its motion.
5. First Law of Motion
• Ex. Think about being in a moving car
that is involved in a front-end collision.
The collision makes the car stop suddenly
but what happens to you, the passenger?
• Because you have inertia, you continue
moving forward.
6. Law of Inertia: Crash Tests
• What forces act on the crash-test dummy
to slow its forward motion?
–Seatbelt: the seatbelt tightens
immediately to slow down the dummy
and to absorb energy.
–Inflating air bag: the bag exerts a force
that slows down the dummy’s forward
motion, absorbing its energy and
prevents it from hitting the steering
wheel.
7. Newton’s Second Law of Motion
• An unbalanced force acting on an object
affects the motion of an object by causing
that object’s velocity to change.
–The object accelerates.
• Newton’s second law explains what
happens to an object when an unbalanced
force acts on it.
• It describes a relationship of the force
acting on an object to the acceleration of
the object.
8. Force vs. Acceleration
• When you throw a ball, you apply a net
force to it.
–Harder you throw it, more it accelerates.
–Acceleration of ball is directly
proportional to the net force acting on it.
–If you double the force acting on the
ball, acceleration of the ball will double.
• THEREFORE…as force increases,
acceleration increases.
9. Mass vs. Acceleration
• Newton also learned that the acceleration
of an object depends on the mass of the
object.
–Mass is a measure of the inertia of an
object and depends on the amount of
matter an object contains.
• Ex. Compare hitting a tennis ball then a
basketball with the same force using a
tennis racket.
• As mass increases, acceleration decreases.
10. Newton’s Second Law
• Second law summarizes, in a
mathematical equation, the relationship
of force, mass and acceleration.
–Acceleration is equal to the net force
acting on an object divided by the
object’s mass.
• Acceleration=net force / mass, or a=F / m
• Can be written solving for force:
Force = mass * acceleration, or F = ma
11. Explore Newton’s Third Law
through the lens of an
AMUSEMENT PARK
• Amusement Park Physics--Bumper Cars!
12. Newton’s Third Law of Motion
• Bumper cars illustrate Newton’s third law.
• The goal is to slam into another car head
on so that it jolts the other driver.
• There are two parts
to this collision
however, because
the collision causes
your car to
rebound sharply
as well.
13. Newton’s Third Law of Motion
• A force cannot exist alone.
• Forces exist in pairs.
• According to Newton’s third law,
whenever one object exerts a force on a
second object, the second object exerts an
equal and opposite force on the first
object.
• These two forces are called action and
reaction forces.
14. Newton’s Third Law of Motion
• The force your bumper car exerts on the
other car is the action force.
• The force the other car exerts on your car
is the reaction force.
• These two forces are equal in size and
opposite in direction.
Action
Force
acting on
other car
Reaction
Force
acting on
your car
15. Newton’s Third Law
• Not all action and reaction forces cause
motion.
• You may be wondering why the
action/reaction forces do not cancel and
produce a net force of zero.
• The reason is that the action/reaction
forces do not act on the same object.
• Only when equal and opposite forces act
on the same object do they result in a net
force of zero.
16. Newton’s Third Law
• Ex. A swimmer uses her arms to push
against the water creating an action force.
–action force causes water to move in the
direction of action force.
• Water exerts an equal and opposite
reaction force on the swimmer pushing her
forward through the water.
• They do not cancel because:
–action force acts on the water.
–reaction force acts on the swimmer.
17. Action/Reaction Force Pairs
• Scenario 1: A rocket
moves in outer space
with gas propelled out
the back
–Action: Gas particles
are thrust out the
back of the rocket
–Reaction: The gas
applies a force on the
rocket
18. Action/Reaction Force Pairs
• Scenario 2: A person jumps out of a boat
–Action: Foot pushing off the boat.
–Reaction:
The boat
pushing on
the person’s
foot.
19. Action/Reaction Force Pairs
• Scenario 3: A player kicks a soccer ball
–Action: Foot puts force on the soccer
ball.
–Reaction:
The soccer
ball exerts
a force on
the foot/
20. Additional Resources
• Newton's Laws of Motion Flashcards
• Newton's First Law Review Questions
• Newton's Second Law Review Questions
• Newton's Third Law Review Questions
21. References/Acknowledgements
• Presentation was developed using the
following textbook:
–Wysession, Frank, & Yancopoulus.
(2008). Physical Science: Concepts in
Action. Prentice Hall.
• Acknowledgements are also extended to
the creators of demonstration videos and
Quizlet flashcards.
