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Air and Air Resistance
Name: Sharazaad C.
Status: student
Age: 13
Location: N/A
Country: N/A
Date: 7/13/2004
Question:
Please explain how molecules in the air cause air
resistance. Why air resistance is larger when a car a travelling faster?
Replies:
Sharazaad,
Even though you cannot see them, air molecules (a mixture of gases, mainly nitrogen and oxygen) are
just like all other molecules inasmuch as they have mass and occupy space. Anything moving through
air must push the molecules out of the way.
The faster the object moves, the faster the air molecules are encountered and the greater are the
number to be pushed aside. This requires a larger force that necessary when the object is moving
slowly. Consider swimming: The faster you swim, the harder it is to push the water molecules out
of the way.
Streamlining a car is a way of making it slip through the air more easily. Cars with narrow or
wedge-shaped front ends and smooth, gentle curves allow it to slip through the air more easily
than bulky things like large trucks with large, boxy, flat front ends. Even something so simple
as keeping a car clean and polished will help it slip through the air more easily.
Regards,
ProfHoff 869
Sharazaad,
Molecules in the air have mass. Even though they are invisible, they are
still there, and they still have to be pushed out of the way for a car to
occupy the space that the air is in. This now should make sense WHY air
resistance even exists.
NOW. Why do greater speeds cause there to be an even greater air
resistance. It is true that there is barely any appreciable air resistance
to a car going 10 MPH versus 60 MPH. But at 60 MPH, even though there is
the same amount of air, the resistance is much greater because the car needs
to move the air out of the way MUCH MUCH QUICKER. I do not know if you can
really speak about viscosity of air (non - liquid). But air really does
behave like a liquid when it comes to resistance. HOW? The air that is
existing on or above the sidewalk WILL ALSO RESIST the air molecule that is
being struck by your car and pushed to the side. SO, this is why at higher
speeds you will have a much greater resistance because you are FORCING THE
AIR to move not at 10 MPH but NEEDING IT to move out of the way at 60 MPH.
You could say that the relationship between air resistance (r) and velocity
(v) is proportional. However, I think I read somewhere that resistance (r)
is ~ velocity (v)^1.5
Please do not quote me on that ... but I am pretty certain I read it in an
Math book for Engineers. So there may be some credence to it.
Hope this helps.
Darin Wagner
Air resistance is caused by the car banging into air molecules and
increasing their momentum in the direction the car is going. Since the car
exerts a forward directed force on the molecule, by Newton's 2nd law (for
every action there is an equal and opposite reaction) the molecule exerts a
force on the car in the backward direction, tending to slow the car.
Of course, a molecule has so little mass, it exerts an extremely tiny force
on the car. However, there are so many molecules that the cumulative effect
can be large. You might also imagine what would happen if molecules were as
large as bowling balls! I think you can picture how the forces could be
very large.
Incidentally, the molecules are moving at speeds much faster than the car is
going and in random directions. So some hit the car from the rear and speed
it up a little (very little). However, one coming from the front slows the
car down by a larger amount because the car's speed is added to the
molecule's speed for a molecule coming from the front and subtracted from
the molecule's speed for one coming from the rear.
Best, Dick Plano, Professor of Physics emeritus, Rutgers University
Hi Sharazaad,
Good question, slightly complicated answer but I think you can handle it.
First, air resistance is a kind of friction. Friction simply being a force
that acts on materials that are in contact with one another, as well as acts
opposite the direction of motion. Example: you want to move a bookcase
without removing the books, (silly idea but go with it), you push in one
direction, it is pretty tough because friction is working against you. You
go out and buy those "super sliders" to move the bookcase. They REDUCE the
friction, it is still there it is just easier for you to OVERCOME the friction
and move your bookcase.
OK, now the car. Air is considered a fluid, (just like water), they
actually work in much the same way. Fluid friction occurs when an object
pushes aside the fluid it is moving through. Think running on land and then
compare it to running through waist deep water. Get the idea? Air
resistance is kind of fluid friction. As the car moves through the air, the
molecules of air are bumping into the car, good thing they do not leave
dents! The car engine applies a force to make the tires rotate and move the
car forward.
Now think of Newton's Third Law, "For every action there is and equal and
OPPOSITE reaction. I bet you were able to finish that sentence without even
having to read it. Friction acts opposite the direction in which the car is
moving. In order for the car to move at a constant speed, the friction
force has to balance the force of the car moving forward. There is more
force needed to move the car to a higher rate of speed, therefore the air
resistance must increase to balance it.
I hope this answers your question. Write back if you need more details.
Happy science!
Martha Croll
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