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Homework Help: Air Resistance calculation

  1. Mar 22, 2005 #1
    I'm doing a data handling course work project. I've calculated various values for resistance and found that resistance increases as the speed of the body increases. I know that as the body is rolling very little of the force is friction, so most of it will be air resistance. i have an idea that the air resistance is increasing because as the body rolls faster it collides with more air particles. is this right and can anyone expand on this for me!

    many thanks
  2. jcsd
  3. Mar 22, 2005 #2
    Ever been in a car and stuck your hand out the window? What happened when you opened your palm fully in comparison to when your hand was parallel to your motion? Think of why that may be.
  4. Mar 22, 2005 #3
    your open hand is hitting more air so you feel it more, but if your changing the velocity is the increase in air resistance because its hitting more air or because its hitting the same amount of air with a greater velocity (I'm half thinking about ideas of relative velocity and momentum and collisions now?).

    I'm confusing myself again!!
  5. Mar 22, 2005 #4
    Well you are right, air resistance does increase with velocity. Think of it this way, you are hitting a bunch of air molecules with a certain velocity 'v1'. Those molecules will then travel back a bit by the force imparted on them by you and hit molecules behind them at a slightly smaller velocity 'v2' and so forth all the way to 'v_n' . Now the force will dissipate to zero pretty quickly in real life but thats the basic principle. Now if you hit those first molecules with a higher velocity, say twice as fast '2*v1', then v2 and v3 and so forth will take a much longer time to dissipate that energy.

    The relationship with dissipating the energy and air resistance is that when you hit the air it creates an imbalance in local pressure, so the system must restore the pressure locally and does so by moving the air molecules around (away from your hand). By going faster you are creating a larger imbalance in pressure, and the result is more molecules jumping around trying to fix the problem.

    What I was trying to say with the open and closed hand, is think how many molecules youd be hitting with your hand when its closed, not very many, since basically the width of your finger is the area thats displacing molecules. If you open yoru hand however, you are hitting a much larger amount of molecules and imparting a much larger force. I gave you a big hint in this paragraph on one of the most important factors of air resistance.
  6. Mar 22, 2005 #5
    Thanks, very helpful!!
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