Newton’s Laws of Motion

Physical Science

9th Grade

Designed to demonstrate each law and encourage students to see their application

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.

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.

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.

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.

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.

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.

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.

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.

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

Explore Newton’s Third Law through the lens of an AMUSEMENT PARK

• Amusement Park Physics--Bumper Cars!

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.

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.

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

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.

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.

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

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.

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/

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

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.