Transcript 3.Notes_Newtons_First_Law

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.
An object at Rest
Will remain at rest unless an unbalanced force
acts upon them
Zzzzzzzzzz
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
Eureka! Mass
http://www.animatedscience.co.uk/flv/
Sample Problem: Inertia
MASS
1. Which object has the greatest inertia?
1. A falling leaf
2. A softball in flight
3. A seated high school student
4. A helium-filled toy balloon
Sample Problem: Inertia
2. Which object has the greatest inertia?
MASS
1. A 5-kg mass moving at 10 m/s
2. A 10-kg mass moving at 1 m/s
3. A 15-kg mass moving at 10 m/s
4. 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
1. 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.
2. Supposing you were in space in a weightless environment, would it require
a force to set an object in motion?
3. 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.