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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Please do not quote me on that ...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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