Ask A Scientist Physics Archive

name         Loretta H.
status       educator
age          20s

Question -   Is there a relationship between air pressure and air
resistance? Recently, I had said that the air resistance acting on a
falling object is actually the air pressure that is acting on the  object.
This did not sound right. Air pressure that is acting on an object is I
know a constant value per square area. However, what I was trying to say
was that since air resistance is actually a force that is acting on the
falling object and it is acting on every area of the object, and  since we
know that Pressure = Force divided by Area, therefore, air resistance is
somehow related to the pressure of air acting on that object. Please do
enlighten me on this. Thanks.
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Air resistance is due to the force exerted on the air by the falling
object to push the air aside to let the object proceed through the
air.  By Newton's third law (for every force there is an equal and
opposite reaction) the air pushes on the object with an equal and
opposite force.  The air comes together behind the object, of course,
but the resulting pressure there is less that the pressure in front of
the object.  The difference in these pressures (times an area, as you
mention) is the cause of the air resistance.

Since, for a stationary object, the air pressure is equal on all sides
of the object, it exerts no net force on the object.  If the air
pressure is increased, the net force on a stationary object is still
zero.  The net force on a moving object will increase due to the fact
that the air is denser and the object has to push more air aside.

The detailed calculation of air resistance is complicated, but the
basic idea, as stated here, is simple.

Best, Dick Plano
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Hi, Loretta !!!
I can only understand this problem by considering
that the bigger the pressure, the more resistance
there will be against the movement of a body.
And that because there will be more air to be
crossed. The behavior of the limit layer surely
will show us that at the front the pressure will
be greater than behind, where greater turbulence
should be expected. If there were no movement
the only force acting on the body is that due to
pressure differences, vertical, from the bottom to
the top. This force is independent from the value
of pressure. If the body moves across the air,
there will be a greater pressure ahead and vacuum
at the tail ( depending upon how big is the speed ).
If a body falls, the resistance will be increasingly
bigger, till it reaches a value where there will be no
more acceleration, or be, constant speed. On the
other extreme, without air, there always will be
acceleration, what means increasing velocity.
When comets reach the earth atmosphere - as
you know - the friction is so high that the tempe-
ture increases and oxygen starts a chemical
reaction and burns the comet. In a planet where
the gravity is bigger than at the earth, there will
be more gases present, and the friction will be
bigger.
best regards
Alcir Grohmann
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You are ok in this as long as you are careful about what you mean by air
pressure.  The pressure that acts to oppose an object moving through
air is not the ambient air pressure.  That pressure exerts the same
force on all sides of the object, so the net force from it is zero.

When an object moves through the air, its motion causes the air
pressure in front to increase while the pressure behind decreases, and
this pressure difference produces a net force on the object.  If the
object suddenly stops moving, it will take a while for the higher
pressure air in front to leak around to the back, and while this is
happening, the object will still feel a net force from the pressure
difference.  So it is the pressure that causes the force.

But drag is more complicated than this because there are other things
that happen as the object moves.  The air and the object are heated,
there is turbulence, jets make a condensation trail, etc.  All of these
things must be "paid for" out of the momentum (and energy) of the
moving object, and any time momentum changes, there is by definition a
force of some kind.

Tim Mooney
========================================================
Loretta,
There is a relation between air pressure and air resistance, but air
resistance and air pressure are not the same thing.  If air pressure were
zero, air resistance would also be zero.  Still non-zero air pressure does
not mean any air resistance is being felt.

Air pressure is from all directions.  Air pressure can be different on
different parts of an object, but in most cases it is quite large all
around.  Air pressure is due to molecules crashing into the object from all
around.

Air resistance is due to the motion of an object through the air.  The
object pushes the air molecules out of the way.  The molecules push back.
Because the air molecules in front get squeezed together more tightly,
pressure in front is greater.  Air molecules in back get a little spread
out, so air pressure in back is less.  The net effect is a force opposite
the direction of motion.

Just as important to air resistance is the shape of the moving object.  A
narrow, pointed object pushes the molecules aside quite easily.  A flat
front must push the molecules harder to get them to the side.  It is like
hammering a sharp nail versus a dull peg into a piece of wood.  For an
arrow, the air molecules in front do not get so tightly squeezed together as
for the dull peg.

Air pressure may be viewed as part of why air resistance exists, but it is
not air resistance itself.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College
========================================================
Air pressure does act on all (exposed) surfaces of an object.  The presence of
air pressure does not depend on the state of motion of an object, or even the
presence of an object.  It is a property of the air alone.

Wind resistance can only be talked about in terms of the resistance to motion
of an object in the air.  The presence of a body to be acted on and the motion
of the air around it creates an increase in the air pressure in front of the
object compared to the air pressure behind the object.  This DIFFERENCE in air
pressure results in a new force resisting the motion of the object, which we
call wind resistance.

Greg Bradburn
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