Background information:
Air resistance force is the force that opposes an object's
motion. Air resistance acts in the opposite direction of the object,
pushing against it. The cause of such resistance boils down to molecules and
atoms colliding with one another. As an object moves, its atoms and molecules
collide with the atoms and molecules in the air; these tiny collisions add up
to one large collision. However, air resistance force is dependent upon velocity.
Air
Resistance and Velocity - Even when you walk, air resistance force works
against you, albeit very gently. Because you are not moving very fast, you
won't even notice it, unless it is a very windy day. When you run, however,
even on a calm day, you notice the effects on your face as the air pushes
against you and rushes by you. The reason is that as you speed up, the
collision of atoms and molecules happens at a faster rate
and with more force. This translates to the air resistance force
gets bigger as the speed or velocity of the object increases.
Air
Resistance and Mass - Only
taking into account body size -- not athletic ability -- smaller people of the
same age can usually run faster than larger people. The reason is the air
resistance force. The faster an object moves, the more air resistance;
similarly, the larger an object, the more air that object must displace to push
through it. This means for a larger person running that he must not only
overcome air resistance that the accelerated velocity causes, but he must also
overcome additional air resistance due to his size and shape.
Air
Resistance and Shape - Cars,
buses, trains, airplanes and even boats must overcome air resistance in order
to get people from point A to point B. Transport vehicles for obvious reasons
are much larger than the average human and must move much faster to make it
worth the time for companies to build them. Therefore transport vehicles must
overcome much larger air resistance forces due to faster velocities and larger
masses. To help accomplish this, manufacturers streamline vehicles in an
attempt to reduce drag. This process, however, is never straightforward and
often designs that seem worthy turn out to produce more drag. Usually such
designs are modified continuously and subjected to multiple wind tunnel tests
to improve efficiency.
Objects
in the Air - For
objects moving in the air, another force impacts air resistance: gravity. Gravity is constantly pushing things back toward the ground, while air resistance pushes up on the
object.
In most cases, the
gravity force is much stronger than air resistance, and eventually the object
hits the ground. For an airplane to counteract gravity, the plane must produce lift to keep it in the air and level.
Planes accomplish this through the aerodynamic design of its wings, thereby
making air resistance work for the plane.
BERNOULLI'S PRINCIPLE: Bernoulli’s principle helps explain that an aircraft can achieve lift because of the shape of its wings. They are shaped so that that air flows faster over the top of the wing and slower underneath.
·
Fast moving
air equals low air pressure while slow moving air equals high air pressure.
·
The high air
pressure underneath the wings will therefore push the aircraft up through the
lower air pressure.
REVIEW:
Falling objects increase their speed
as they fall, because their weight (the force of gravity) pulls them to
Earth. They also experience an upward force called air resistance
(drag), which slows them down.
Objects fall faster until they reach their
terminal speed, which is reached when the upward (air resistance) and
downward (weight) forces are equal.
Falling objects
When an
object is dropped it gets faster and faster as it falls. This happens because
their weight (the force of gravity) pulls them down towards the
center of the Earth.
As they fall
through the air, they also experience an upward force called air resistance
(drag).
·
Objects with
large surface areas, such as parachutes or shuttlecocks fall more slowly
because they experience more air resistance.
Frictional forces such as air resistance, friction and
drag act against the
direction of motion, so tend to slow
the object down. This fact is put to
good use in the design of the parachute and shuttlecock.The size of frictional forces can be reduced by streamlining the object or lubricating any moving parts.
Reducing frictional force
Examples of reducing frictional force by streamlining include:- A sports car is wedge-shaped to reduce air resistance and so increase top speed.
- Lorries and caravans have deflectors to reduce both air resistance and fuel consumption.
- A car with a roof box has increased air resistance and fuel consumption and a lower top speed.
- A downhill skier puts wax on the skis to reduce friction and so increase the top speed.
Terminal speed
When objects fall through the Earth's atmosphere they get faster and faster until they reach a speed where the upwards force (air resistance) and downwards force (weight) equal each other. At this point the object travels at its fastest speed called terminal speed.
What happens
when you drop a coin and small feather at the same time?
·
They both
have a similar surface area
but the feather weighs less
so has a smaller force of gravity
pulling it down.
·
As the
feather falls its upwards air resistance increases
and soon equals its
downwards weight, so it then travels at terminal speed. The coin is heavier and has to be traveling
a lot faster before its
air resistance is large enough to equal its weight.
Terminal speed on the moon
The
astronaut, David Scott carried out a famous experiment on the Moon. He dropped
a hammer and a feather at the same time and found they landed together. As there is no air on the Moon there is no air resistance. The only
force on both the hammer and feather was the Moon’s gravity which made them both
fall with the same acceleration.
Balanced forces - The motion of a skydiver
Graph showing the motion of a skydiver
Before the parachute opens
1. When the skydiver jumps out of the plane he accelerates due to the force of gravity pulling him down.2. As he speeds up the upwards air resistance force increases. He carries on accelerating as long as the air resistance is less than his weight.
3. Eventually, he reaches his terminal speed when the air resistance and weight become equal. They're said to be balanced.
After the parachute opens
4. When the canopy opens it has a large surface area which increases the air resistance. This unbalances the forces and causes the parachutist to slow down.5. As the parachutist slows down, his air resistance gets less until eventually it equals the downward force of gravity on him (his weight). Once again the two forces balance and he falls at terminal speed. This time it's a much slower terminal speed than before.
