Download Galileo

Newton’s
nd
2 Law
Air Resistance & Drag
Why does the motion of
objects change?
Aristotle
Objects seek their ‘natural’ places
OR
‘Unnatural’ forces act on those objects
Greek philosopher,
~2400 BCE.
For more, start here.
Why does the motion of
objects change?
Aristotle
Objects seek their ‘natural’ places
OR
‘Unnatural’ forces act on those objects
Galileo
Challenged Aristotle’s ideas with “thought
experiments” and investigations.
Italian philosopher,
~ 1600.
For more, start here .
Galileo’s
thought experiment #1
• On rough surfaces, a rolling
ball will stop quickly.
• On a smooth surface, a rolling
ball will travel much farther.
• On a PERFECTLY smooth
surface, how far will a rolling
ball travel?
Galileo’s
thought experiment #2
A ball rolling uphill will come to stop.
A ball rolling downhill will speed up.
In the absence of friction, at what
angle will the ball neither speed up
NOR slow down?
Galileo’s big idea #1:
Any change in direction or
speed requires an outside
force!
– You, me, horse, exploding
gasoline, etc.
– Friction, air resistance
Inertia
Inertia is the resistance of an
object to a change in motion, an
intrinsic property of matter
Latin
In – without
erg – force
Objects in outer space are
weightless, but still have inertia;
related to “momentum”
NOT the same as weight
Galileo’s
thought experiment #3
Heavy things fall faster than
lighter things. If you have a
block that weighs twice as
much as another block, does
the heavy block fall twice as
fast as the lighter block?
Galileo’s
thought experiment #3
• What happens to the rate
at which the heavy block
falls if you cut it in half and
connect the two pieces
with a string?
???
Galileo’s
actual experiment #1
• Rolled balls down inclined planes
– Timed first with pulse
– Later timed with water clock
• Distance travelled discovered to be
proportional to the SQUARE of the
time elapsed
distance  time2
– NOT proportional to mass
Galileo’s big idea #2:
Weight does not affect
rate at which something
falls; air resistance does.
•Dave Scott on the moon
What causes the motion of
objects to change?
Newton
We can describe the motion of
objects using three principles:
1st Inertia
An object in straight-line motion will
stay in straight-line motion until acted
upon by an unbalanced force.
An object at rest will stay at rest until
acted upon by an unbalanced force.
Balanced forces
5N
5N
0N
Describe the
motion of the
block if the
forces are
balanced.
Unbalanced forces
5N
5N
10 N
What causes the motion of
objects to change?
Newton
F
m
a
 a

F
a=
m
We can describe the motion of
objects using three principles:
1st
Inertia
2nd Acceleration is proportional to
force.
Acceleration is inversely
proportional to mass.
alternatively,
F = ma
Think about it…
If you push on a shopping cart, it
will accelerate.
1) If you push 5 x harder, what
happens to its acceleration?
2) If you push the same, but the cart is
loaded so that it has 5 x the mass,
what happens to the acceleration?
3) If you push 5 x harder when its
mass is 5 x greater, what happens
to the acceleration?
What causes the motion of
objects to change?
Newton
We can describe the motion of
objects using three principles:
1st
2nd
Inertia
Acceleration is proportional to force and
inversely proportional to mass
3rd If one object acts on a second,
the second acts on the first
with equal magnitude in the
opposite direction.
For every action,
there is an equal and
opposite reaction.
alternatively, forces
come in pairs.
Newton’s Laws of Motion
To summarize:
1st Inertia
2nd a = F / m
3rd Action & reaction
A note about units of force
In the metric system,
• mass is measured in
kilograms (kg)
• acceleration is measured
in (meters per second)
per second, (m/s/s OR
m/s2)
In the Imperial system,
• mass is measured in slugs
• acceleration is measured in (feet
per second) per second, (ft/s/s
OR ft/s2)
Since force is equal to mass (kg)
x acceleration (m/s / s), the units
of force are kg  m/s/s.
Pronounced: Newton
Abbreviated: N
In the Imperial system,
• force is measured in pounds (lbs)
Modeling Forces of Flight
Drag
Drag slows a plane down
– Drag acts opposite to the
direction of flight
– Drag limits the forward
speed of the airplane
Types
Parasitic drag
Induced drag
Influenced by how air
flows around entire
aircraft.
Influenced by the
direction air is deflected
by aircraft.
Parasitic Drag
Caused by surfaces which deflects or interferes with
the smooth airflow around the airplane
Size & shape
Interference
Skin
Induced drag
Airfoils deflect
air downward
and rearward.
Induced drag
Downward deflection of air pushes plane upward
Rearward deflection of air causes induced drag.
Flaps
Flaps
Flaps
Flaps
• Flaps change the shape of the
wing to deflect more air.
• Some of that deflected air
creates goes down
(and pushes the plane up).
• Some of that deflected air
pushes the plane backwards
(and slows the plane down).
Think about it…
Flaps generate more lift
• Allow aircraft to fly at
slower speeds
Flaps generate more drag
• Allow aircraft to slow down
In what situation(s)
is it useful to fly at
slower speeds?
Think about it…
As a token of appreciation, you
take your 9th grade science
instructor skydiving on your 18th
birthday.
1) The moment you leap from
the plane, air resistance is
~0 N. At what rate does your
speed change when air
resistance is ~0 N?
2) After a while, you stop
accelerating. Why?