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Name: Loretta H.
Status: educator
Age: 20s
Location: N/A
Country: N/A
Date: 2001-2002


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.


Replies:
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


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


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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