Newton’s Laws of Newton’s Laws of MotionMotion

ObjectivesObjectivesDefine forceDefine force

Define mass and inertiaDefine mass and inertia

Explain the meaning of Newton’s 1Explain the meaning of Newton’s 1stst Law Law

Let’s start with a Magic Let’s start with a Magic Trick?Trick?

Predictions:Predictions:

Observations:Observations:

Hmm, how does physics explain this Hmm, how does physics explain this trick?trick?

Change of Plans…..Change of Plans…..http://www.youtube.com/watch?v=ZFoG7HRF2mE&safety_mode=true&persist_safety_mode=1

Newton’s First Law of Newton’s First Law of MotionMotion

An object at rest will remain at rest, and an An object at rest will remain at rest, and an object in motion will remain in motion, at object in motion will remain in motion, at constant velocity in a straight line, unless constant velocity in a straight line, unless acted upon by a net (unbalanced) force. acted upon by a net (unbalanced) force.

(Also known as the law of inertia)(Also known as the law of inertia)

What does it mean?What does it mean?Whatever an object is doing, thatWhatever an object is doing, that’’s what s what it wants to keep doing.it wants to keep doing.

If left to themselves, objects will keep If left to themselves, objects will keep doing whatever theydoing whatever they’’re doing.re doing.

An object at RestAn object at Rest

Will remain at rest unless an Will remain at rest unless an unbalanced force acts upon themunbalanced force acts upon them

Zzzzzzzzzz

Objects in MotionObjects in MotionWill remain in motion at Will remain in motion at constant velocity unless constant velocity unless acted upon by an acted upon by an unbalanced forceunbalanced force

Much less obviousMuch less obvious

Hard to find a frictionless Hard to find a frictionless environment here on Earthenvironment here on Earth

Picture yourself in space and Picture yourself in space and throw a wrenchthrow a wrench

What would happen to that What would happen to that wrench?wrench?

Eureka! InertiaEureka! Inertia

http://www.animatedscience.co.uk/flv/

InertiaInertiaInertiaInertia is the tendency of an object to is the tendency of an object to resist a change in velocityresist a change in velocity

Mass actually has two aspectsMass actually has two aspects

Inertial Mass Inertial Mass is how hard it is to is how hard it is to change an object’s velocitychange an object’s velocity

Gravitational massGravitational mass is how strongly a is how strongly a gravitational field affects a massgravitational field affects a mass

For the purposes of basic into physics, For the purposes of basic into physics, massmass andand inertiainertia are synonymousare synonymous

Eureka! MassEureka! Mass

http://www.animatedscience.co.uk/flv/

Sample Problem: InertiaSample Problem: Inertia

1. Which object has the greatest inertia?1. Which object has the greatest inertia?

1.1. A falling leafA falling leaf

2.2. A softball in flightA softball in flight

3.3. A seated high school student A seated high school student

4.4. A helium-filled toy balloonA helium-filled toy balloon

MASS

Sample Problem: InertiaSample Problem: Inertia

2. Which object has the greatest inertia?2. Which object has the greatest inertia?

1.1. A 5-kg mass moving at 10 m/sA 5-kg mass moving at 10 m/s

2.2. A 10-kg mass moving at 1 m/sA 10-kg mass moving at 1 m/s

3.3. A 15-kg mass moving at 10 m/sA 15-kg mass moving at 10 m/s

4.4. A 20-kg mass moving at 1 m/sA 20-kg mass moving at 1 m/s

MASS

Back to the MagicBack to the Magic

Now can you explain the magic trick and the Now can you explain the magic trick and the physics involvedphysics involved

What about this other magic trick?What about this other magic trick?

Newton’s First Law and Newton’s First Law and FootballFootball

http://science360.gov/obj/video/70fadaa8-c3d4-4132-ba1f-c98be5caeb14

Understanding CheckUnderstanding Check1.1. Mac and Tosh are arguing in the cafeteria. Mac says that if Mac and Tosh are arguing in the cafeteria. Mac says that if

he flings the Jell-O with a greater speed it will have a he flings the Jell-O with a greater speed it will have a greater inertia. Tosh argues that inertia does not depend greater inertia. Tosh argues that inertia does not depend upon speed, but rather upon mass. Who do you agree with? upon speed, but rather upon mass. Who do you agree with? Explain why.Explain why.

2.2. Supposing you were in space in a Supposing you were in space in a weightless environmentweightless environment, , would it require a force to set an object in motion?would it require a force to set an object in motion?

3.3. Fred spends most Sunday afternoons at rest on the sofa, Fred spends most Sunday afternoons at rest on the sofa, watching pro football games and consuming large quantities watching pro football games and consuming large quantities of food. What effect (if any) does this practice have upon his of food. What effect (if any) does this practice have upon his inertia? Explain.inertia? Explain.