Transcript Law of Inertia

Aristotle
◦ First to study motion

Galileo
◦ Dropped big and small rock  Fell
at same rate
◦ If no interference – motion will go
on forever

Newton formalized ideas
◦ At 23 (1665) he overthrew
Aristotle’s 2000 year old
ideas
◦ LAWS OF MOTION

Force
◦ The cause of an acceleration (or the change in an
object’s motion)
◦ Force = mass x acceleration
◦ Units:
 Newton – SI Unit of Force (kg x m/s2)
 Dyne – CGS Unit of Force (g x cm/s2)
 Pound – English Unit of Force (slug x ft/s2)

Inertia
◦ The tendency of an object to maintain its state of
motion

Equilibrium
◦ The state of a body in which there is no change in
its motion

Weight
◦ The magnitude of the force of gravity acting on an
object
◦ Weight = mass x acceleration

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

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?
Zigzag!!
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)

Does mass = weight?
◦ Mass – quantity of matter in a body
◦ Weight – measure of gravitational force which acts
on a body  W = mg
 Depends on where the object is
 Ex. Earth, Moon, Space
g = 9.8 m/s2
g = 1.6 m/s2
g=?

Gravity on the Moon
◦ http://www.youtube.com/watch?v=PE81zGhnb0w

Gravity around the Solar System
◦ http://www.aerospaceweb.org/question/astronomy
/q0227.shtml

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

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?

Dishes on Table (rest  rest)
◦ Snap a cloth from beneath
◦ Friction between dishes and cloth not enough to
move dishes

In stopping car (motion  motion)
◦ Fly forward
◦ Keep moving while car stops


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
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


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.


“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
mks
cgs
English
Length
m
cm
ft
Mass
kg
g
slugs
Time
s
s
s
Velocity
m/s
cm/s
ft/s
Acceleration
m/s2
cm/s2
ft/s2
Dyne
(g x cm/s2)
Pounds (lbs)
(slugs x ft/s2)
Force
Newtons (N)
(kg x m/s2)
mass x gravity
W = N or lbs g =
-9.8 m/s2 or -32 ft/s2
N or lbs
Use ______________
as force units
push or a __________
pull
A force is a __________
Now also say: Force is anything that can ____________
accelerate a body.
Weight =


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

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

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

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.