This principle is used by seat belts because they apply a force opposite to the passenger’s forward motion. The force decelerates the passenger to prevent serious injury.
Newton's Laws of Motion
Newton’s Laws of MotionPhysical Science9th GradeDesigned to demonstrate each law andencourage students to see their application
Sir Isaac Newton• Newton built on thework of scientists suchas Galileo.• Published his worksafter many years in abook entitles Principia.• He first had to defineforce and mass. Hethen defined his lawsof motion.
Newton’s First Law of Motion• Newton’s first law of motion states thatthe motion of an object does not changeas long as the net force acting on theobject is zero.• Thus, unless an unbalanced force acts, anobject at rest remains at rest, and anobject in motion remains in motion withthe same speed and direction.
Newton’s First Law of Motion• Often called the Law of Inertia.–Inertia is the tendency of an object toresist a change in its motion.
First Law of Motion• Ex. Think about being in a moving carthat is involved in a front-end collision.The collision makes the car stop suddenlybut what happens to you, the passenger?• Because you have inertia, you continuemoving forward.
Law of Inertia: Crash Tests• What forces act on the crash-test dummyto slow its forward motion?–Seatbelt: the seatbelt tightensimmediately to slow down the dummyand to absorb energy.–Inflating air bag: the bag exerts a forcethat slows down the dummy’s forwardmotion, absorbing its energy andprevents it from hitting the steeringwheel.
Newton’s Second Law of Motion• An unbalanced force acting on an objectaffects the motion of an object by causingthat object’s velocity to change.–The object accelerates.• Newton’s second law explains whathappens to an object when an unbalancedforce acts on it.• It describes a relationship of the forceacting on an object to the acceleration ofthe object.
Force vs. Acceleration• When you throw a ball, you apply a netforce to it.–Harder you throw it, more it accelerates.–Acceleration of ball is directlyproportional to the net force acting on it.–If you double the force acting on theball, acceleration of the ball will double.• THEREFORE…as force increases,acceleration increases.
Mass vs. Acceleration• Newton also learned that the accelerationof an object depends on the mass of theobject.–Mass is a measure of the inertia of anobject and depends on the amount ofmatter an object contains.• Ex. Compare hitting a tennis ball then abasketball with the same force using atennis racket.• As mass increases, acceleration decreases.
Newton’s Second Law• Second law summarizes, in amathematical equation, the relationshipof force, mass and acceleration.–Acceleration is equal to the net forceacting on an object divided by theobject’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 Lawthrough the lens of anAMUSEMENT 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 headon so that it jolts the other driver.• There are two partsto this collisionhowever, becausethe collision causesyour car torebound sharplyas 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 asecond object, the second object exerts anequal and opposite force on the firstobject.• These two forces are called action andreaction forces.
Newton’s Third Law of Motion• The force your bumper car exerts on theother car is the action force.• The force the other car exerts on your caris the reaction force.• These two forces are equal in size andopposite in direction.ActionForceacting onother carReactionForceacting onyour car
Newton’s Third Law• Not all action and reaction forces causemotion.• You may be wondering why theaction/reaction forces do not cancel andproduce a net force of zero.• The reason is that the action/reactionforces do not act on the same object.• Only when equal and opposite forces acton the same object do they result in a netforce of zero.
Newton’s Third Law• Ex. A swimmer uses her arms to pushagainst the water creating an action force.–action force causes water to move in thedirection of action force.• Water exerts an equal and oppositereaction force on the swimmer pushing herforward 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 rocketmoves in outer spacewith gas propelled outthe back–Action: Gas particlesare thrust out theback of the rocket–Reaction: The gasapplies a force on therocket
Action/Reaction Force Pairs• Scenario 2: A person jumps out of a boat–Action: Foot pushing off the boat.–Reaction:The boatpushing onthe person’sfoot.
Action/Reaction Force Pairs• Scenario 3: A player kicks a soccer ball–Action: Foot puts force on the soccerball.–Reaction:The soccerball exertsa force onthe foot/
Additional Resources• Newtons Laws of Motion Flashcards• Newtons First Law Review Questions• Newtons Second Law Review Questions• Newtons Third Law Review Questions
References/Acknowledgements• Presentation was developed using thefollowing textbook:–Wysession, Frank, & Yancopoulus.(2008). Physical Science: Concepts inAction. Prentice Hall.• Acknowledgements are also extended tothe creators of demonstration videos andQuizlet flashcards.