Transcript Newton`s First Law - Inertia

Newton’s First Law - Inertia
Chapter Four
Aristotle on Motion
Greek scientist – 400 BC
Two types of Motion
– Natural – Straight-up motion (smoke) or
straight-down motion (falling) or circular
(planets)
Objects would seek their “natural” resting
places
Not thought to be caused by forces
Aristotle on Motion
– Violent – imposed
motion
From push or pull
External causes
Examples – cart, tug-ofwar
Objects in natural resting
positions would not move
by themselves
Copernicus and Moving Earth
Astronomer – 1473-1543
Assumed earth and
planets moved around the
sun
Used to think earth was
the center of the universe
Published his findings
around the time of his
death
Galileo on Motion
Believed
Copernicus
Was put under
house arrest by
the church
Biggest
contribution –
force in NOT
needed to keep an
object moving
Force and Friction
Force – push or pull
Friction – force
between materials that
touch
– Caused by surface
irregularities
– Friction stops motion
Galileo’s Experiments
Only when friction is present is a
force needed to get something
moving
Tested his idea with rolling balls on
plane surfaces at different angles.
Rolling down a hill,
the speed
increases
Galileo’s experiments
Rolling up a hill,
speed
decreases
He reasoned the
without friction –
the ball would
continue forever
No slope,
speed
remains
constant
Galileo’s experiments
The balls would rise up on each
side the same height
So what would happen if it was
flat?
The ball would go on forever….
Inertia
A moving body
keeps moving
Galileo
– Concerned with
how it worked
– not why
– Experiments
(not logic)
leads to
knowledge
Sir Isaac Newton
Born Christmas day – in the same
year as Galileo’s death
Had made his famous laws by the
time he was 24.
1st law of Motion - Inertia
Every object continues in a state of
rest, or of motion in a straight line at
constant speed, unless it is
compelled to change by forces
exerted on it
Examples – coin on paper, dishes on
table, hockey puck on air table,
Pioneer and Voyager
Question
If the force of gravity
between the sun and
planets suddenly
disappeared, what type of
path would the planets
follow?
Is it correct to say that
the reason an object
resists change and persist
in its state of motion is
that it has inertia?
Answer
The planets would move in straight lines
at constant speed (velocity)
No, we don’t know the reason why objects
persist in their motion when nothing acts
on them, but we call this property inertia.
We understand many things – but some
things – gravity and inertia for example,
are still unexplained.
Inertia
Mass – a Measure of Inertia
Kicking a can
– Empty
– Filled with toothpicks
– Filled with nails
Amount of inertia is
related to mass of
object
Mass is NOT volume
Volume – is space
– Cubic centimeters, cubic meters, liters
Which weighs more
– 1 kg of feathers
– 1 kg of nails?
Which has more volume?
Mass is not Weight
Mass – amount of matter, depends
on number and kind of atoms
Weight – force from gravity acting on
it
They are proportional to each other –
but not equal to each other
Question
Does a 2 Kg iron block have twice as
much inertia as a 1 kg block of iron?
Twice as much mass?
Twice as much volume?
Twice as much weight (in the same
location?)
Answer
Yes to all questions. It has twice as
many atoms, so amount of matter,
mass, weight will all increase.
Question
Does a 2-kg bunch of bananas have
twice as much inertia as a 1 kg loaf
of bread?
Twice as much mass?
Twice a much volume?
Twice as much weight (when
weighed in same location)
Answer
Two kilograms of anything will have
twice the mass and inertia. In the
same location, 2kg will have twice
the weight as 1kg. However, volume
and mass are related by density.
Bananas are much more dense than
bread, therefore they will occupy less
space.
Weight
In USA – measured in pounds
In World – measured in
kilograms
1 Kg (mass) = 2.2 pounds
(weight)
How many kilograms do you
weigh?
– Your weight x 1 kg/2.2 pound =
Force
A push or a pull
Measured in Newtons
1 N is about the weight of a cooked
burger
1 kg weighs 9.8 N
– 1 kg = 9.8 N = 2.2 lbs (on earth)
Weight = mass X acceleration
F = ma or F = mg (gravity)
Question
1 kg of nails weighs 9.8 N at Earth’s
surface. Does 1 kg of yogurt also
weigh 9.8 N at Earth’s surface?
Answer
Yes, anything that masses 1 kg will
have a weight of 9.8 N.
Net Force
According to 1st law – object either
stay at rest or stay moving – unless
a force changes it
5N
5N
Is this block at rest?
Net Force
Combination of all forces acting on
an object
– If the net force is equal to zero then
there is NO motion
5N
5N
Net Force Combination
10 N
5N
5N
The two applied forces work together
to make a net force of 10 N
Equilibrium
When net forces equal zero
Example-Forces on a book on a table
– Gravity
– Support force from the table
Also called normal force
Caused by the atoms in the table “pushing”
back
The book compresses the atoms (like a
spring)
– The table pushed back with the same
force that gravity pulls down
Equilibrium
Example – Hanging from a rope
– Atoms are not compressed – they are
stretched
– The force of tension = force of weight
What if hanging by two ropes?
– The tension is divided equally by the
ropes
Question
When you step on a bathroom scale,
the downward force by your feet and
the upward force supplied by the
floor compress a calibrated spring.
In effect, the scale measures the
floors support force. What will the
scale read if you stand on 2 scales?
Answer
The scales will add up to your
weight. If you balance your weight,
the scales will read the same. If you
shift your weight to one side, the
scales will have different weights,
but will still add up to your entire
weight.
Using force scales
10 N
10 N
5N
10 N
5N
The scales
are in
equilibrium
The
resultant of
the forces
equal the
downward
pull
Vector Addition of Forces
Non-vertical orientation of forces
– Tension is greater
– Depend on angle from the vertical
Use the same vector adding to
calculate resultant forces
Spring scales at an Angle
10 N
60 60
10 N
Clothesline
Why can a clothesline support you
vertically but not horizontally?
50 N
50 N
50 N
50 N
Clothesline
Use the
parallelogram
method to find the
force
Resultant
50 N
50 N
•Note - as the rope
becomes more
horizontal resultant
force would become
close to infinity!!
The Moving Earth
When Copernicus first announced the
idea that the Earth moved around
the sun at 30 km/sec, people were
skeptical
Reason – How can a bird jump down
out of a tree and catch a worm if the
earth moves 30 km/sec while the
bird is flying?
Refute
The whole environment is moving at
30 km/sec
– Tree
– Worm
-Bird
-Air
400 years ago people had trouble
with high speeds
Today we know the coins flip the
same in a car or on the ground
Conclusion
Aristotle – didn’t recognize inertia
because he thought there where
different rules for heaven and earth
Galileo and Newton – Recognized
that all moving things follow the
same rules
– Things keep on moving unless a force
(friction) stops it