The Student Room Group
Reply 1
Only Me
Hello!!!

Can anyone tell me why air resistance increases as the velocity of the body increases.

i can't seem to find any useful websites and well i'm stuck.

Thank you
xxx
Air resistance is a reaction force to the driving force of the body so as the forward force is larger so is the resistance
Reply 2
air resistance is the reaction force to the thrust/ motive force as according to Newton's thrid law of motion every action has an equal but opposite reaction,I think it's directly proportional to the thrust force when the two forces are balanced. It increases because if you imagine-as the body travels faster and faster in air for example, it will collide with a higher number of air molecules which in effect act in the opposite direction to it, so if the velocity or thrust force is increased there will be more collisions with air moecules so that force increases to oppose the thrust force. ( probably very confusing-sorry it's been a long day!!!) in simpler terms the faster you go, the larger the force against you due to collisions with air molecules.
Reply 3
Fluid dynamics is a very complex subject and I don't pretend to understand it completely. Luckily, most of the time the drag forces (of air resistance etc.) when an object moves through a fluid are broken down into two: 1) the laminar flow of air around the object, where the drag force is proportional to velocity and 2) the turbulent flow of air as eddies are created, where the drag force is proportional to the square of the velocity. Although the collisions with the air molecules are the obvious cause, I don't think you would get the right answer if you just used simple collisions calculations. The model I'm aware of is more to do with the pressure gradient created.

When an object is travelling through a fluid, it increases the pressure of the fluid in front of it and creates a lower pressure behind it (producing a slip-stream or near-vacuum in some cases). On a basic level, it's this difference in pressures that means there's a greater force acting on the front of the object than on the back, and so the resultant is in the opposite direction to the motion. The faster the object is travelling, the greater the pressure difference and the greater the resultant force. I think the constant of proportionality contains information about the shape of the object, and the viscosity of the fluid. Stokes' force and Archimedes upthrust probably have something to do with it as well.

Just to nitpick, as well...

- Air resistance is not a reaction force, although it is tempting to think of it as one. A reaction force, as the name suggests, "reacts" against some other force. You can have air resistance without any other forces acting. For instance, if you slid an object very fast over a very smooth surface (with almost no friction from the surface, such as ice) then once it leaves your hand, the only force acting would be air resistance.

- When the thrust and drag forces on an object are equal, then of course they are proportional to each other: the constant of proportionality is one! The object reached its so-called "terminal speed". (Although theoretically it will never actually reach terminal speed, it will just become very very very much closer to it as time progresses, assuming the thrust force is constant)



Mmmmm...early morning physics - I like it. I'm really just putting off my revision though!