Newton’s Laws of Motion
History
Aristotle


First to study motion
Galileo


Dropped big and small rock 
Fell at same rate
If no interference – motion will go
on forever
History

Newton formalized ideas


At 23 (1665) he overthrew
Aristotle’s 2000 year old
ideas
LAWS OF MOTION
Newton’s First Law of Motion

Law of Inertia

An object continues in its state of rest, or of
uniform motion in a straight line, unless it is acted
upon by an unbalanced force

“Object” – any body

“Continues” – keeps rest or moving

“Unbalanced force” – net force, not in equilibrium
Law of Inertia

Why Law of Inertia?

Tendency of a body to resist
change of motion


More matter = more inertia
Ex. If chased by an elephant,
how should you run away?
Zig-zag!!
Mass and Inertia
V I
M
V
Cardboard
I M
Wood
V
I M
Lead
• Same volumes  take up the same amount of space
• Which is harder to move?
- More inertia
- More mass (mass is a measure of inertia)
Mass and Weight

Does mass = weight?


Mass – quantity of matter in a body
Weight – measure of gravitational force which
acts on a body


Depends on where the object is
Ex. Earth, Moon, Space
g = 9.8 m/s2
g = 1.6 m/s2
g=?
Mass and Weight

Gravity on the Moon


http://www.youtube.com/watch?v=PE81zGhnb0w
Gravity around the Solar System

http://www.aerospaceweb.org/question/astronomy
/q0227.shtml
Mass and Weight

How much would a 75 kg (165 lb) person
weigh on the Moon? On Mars?






gmoon= 1.62 m/s2
gmars= 3.77 m/s2
2.2 pound = 1 kilograms
1 pound = 4.45 Newtons
Moon: 121.5 N or 27.3 lbs
Mars: 282.8 N or 63.5 lbs
Examples of Inertia

On bus (rest  rest)




Standing and bus moves forward
You seem to fall backward
Actually feet move forward with bus and body
remains at rest
Bus stops?
Examples of Inertia

Dishes on Table (rest  rest)


Snap a cloth from beneath
Friction between dishes and cloth not enough to
move dishes
Examples of Inertia

In stopping car (motion  motion)


Fly forward
Keep moving while car stops
Newton’s Second Law of Motion


Deals with acceleration
Acceleration = a = v / t

Ex. Car died  tow
5N
50 N
Which provides the greater
acceleration?
Which can get car from 0 mi/hr to
20 mi/hr faster?
GREATER FORCE
Newton’s Second Law of Motion
With same force, which can go from 0
mi/hr to 50 mi/hr faster?
Mass = 1000 kg
SMALLER MASS
SMALLER INERTIA
Mass = 5000 kg
Second Law

Force = mass x acceleration (F = ma)

The more force on an object, the more it
accelerates. The more massive an object,
the more it resists acceleration.
Second Law


“The acceleration of a body is directly proportional
to the magnitude of the net force, in the same
direction as the net force, and inversely
proportional to the mass of the body”
* “in the same direction as the net force”




a in the same direction of body’s motion  speed up
a in opposite direction of body’s motion  slow down
a at right angles to direction of body’s motion  deflect
circular
Any other  change in speed and direction
Units
mks
cgs
English
Length
Mass
Time
Force
Weight =
W=
g=
Use ______________ as force units
A force is a __________ or a __________
Now also say: Force is anything that can ____________ a body.
History Revisited


Galileo gave no reason why bodies fall with
an acceleration.
Newton did.


Newton – a F pulls on a body and pulls it toward
the Earth.
F = ma


Heavier body attracted with more F.
Heavier body has more m and…



More I
More resistance to change
Ex. If 2 x F and 2 x m  same net acceleration
Newton’s Third Law of Motion



Force:
 Causes acceleration
 A push or a pull
 Always involves two objects 
interaction
Ex. Hammer and Nail
 Hammer exerts force on nail
 In process, nail slows down hammer
 Therefore, nail exerts force on
hammer
Ex. Ice skater with rifle or with mitt
Newton’s Third Law of Motion

To every action there is an equal and
opposite reaction


One force is action force, other is reaction
Neither can exist without the other
Action
Reaction
Weightlifter lifts on barbell
Barbell pulls on weightlifter
Feet down on Earth
Earth pushes up on feet
You sit on chair
Chair pushes up on you
Tires sit on road
Road pushes up on tires
You push on wall
Wall pushes back on you
Helical blades push down on particles
Particles push up on blades
Rifle pushes bullet
Bullet pushes back on rifle (KICK)
SAME FORCE:
F/mb = a
F/mr = a
Newton’s Third Law of Motion

Sometimes not so obvious.

Reactions in free-fall




Earth pulls on body = action
Body pulls on Earth = reaction
Therefore, can say boulder falls to Earth or Earth falls to
boulder
F is equal but acceleration is very small


When step off curb – street comes up to meet you
Bug against windshield – deceleration of car small,
deceleration of bug large

When objects are relatively the same mass,
both forces are more easily noticed since
both accelerations are about equal.