Newton’s Laws of Motion

Objectives Define force Define mass and inertia Explain the meaning of Newton’s 1st Law

Let’s start with a Magic Trick? Predictions: Observations: Hmm, how does physics explain this trick?

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

Newton’s First Law of Motion An object at rest will remain at rest, and an object in motion will remain in motion, at constant velocity in a straight line, unless acted upon by a net (unbalanced) force. (Also known as the law of inertia)

What does it mean? Whatever an object is doing, that’s what it wants to keep doing. If left to themselves, objects will keep doing whatever they’re doing. Tough concept to see on Earth because its so counter intuitive to what we experience on earth

An object at Rest Will remain at rest unless an unbalanced force acts upon them Zzzzzzzzzz Need a picture of an object at rest

Objects in Motion Will remain in motion at constant velocity unless acted upon by an unbalanced force Much less obvious Hard to find a frictionless environment here on Earth Picture yourself in space and throw a wrench What would happen to that wrench?

Eureka! Inertia http://www.animatedscience.co.uk/flv/

Inertia Inertia is the tendency of an object to resist a change in velocity Mass actually has two aspects Inertial Mass is how hard it is to change an object’s velocity Gravitational mass is how strongly a gravitational field affects a mass For the purposes of basic into physics, mass and inertia are synonymous Change in velocity = acceleration Inertial mass- how hard it is to accelerate an object Anywhere we measure the two, they always turn out to be the same!

Eureka! Mass http://www.animatedscience.co.uk/flv/

Sample Problem: Inertia MASS 1. Which object has the greatest inertia? A falling leaf A softball in flight A seated high school student A helium-filled toy balloon

Sample Problem: Inertia MASS 2. Which object has the greatest inertia? A 5-kg mass moving at 10 m/s A 10-kg mass moving at 1 m/s A 15-kg mass moving at 10 m/s A 20-kg mass moving at 1 m/s

Back to the Magic Now can you explain the magic trick and the physics involved What about this other magic trick?

Newton’s First Law and Football http://science360.gov/obj/video/70fadaa8-c3d4-4132-ba1f-c98be5caeb14

Understanding Check 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 greater inertia. Tosh argues that inertia does not depend upon speed, but rather upon mass. Who do you agree with? Explain why. Supposing you were in space in a weightless environment, would it require a force to set an object in motion? Fred spends most Sunday afternoons at rest on the sofa, watching pro football games and consuming large quantities of food. What effect (if any) does this practice have upon his inertia? Explain.