Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Air resistance, how to calculate it?

  1. Nov 17, 2003 #1
    Im taking AP Physics as well as Calculas but haven't covered the topic of Air Resistance yet. Im trying to figure out what the air resistance would be on a baseball. Could someone help me out?
    Much thanks
  2. jcsd
  3. Nov 17, 2003 #2


    User Avatar
    Science Advisor
    Gold Member


    [itex] F_D = \frac{C_D\rho A v^2}{2} [/itex]

    where FD is the drag force, CD is the drag coefficient, ρ is the density of air, A is the cross-sectional area of the ball (a regulation baseball has a circumference between 9 and 9.25 inches), and v is the velocity of the ball. The drag coefficient is a function of things like surface roughness, ball speed, and spin, varying between 0.2 and 0.5 for speeds commonly occuring during game play, but if you'd like to ballpark it (pardon the pun), 0.3 is a good number to use.
  4. Nov 17, 2003 #3


    User Avatar
    Staff Emeritus
    Gold Member

    What level is AP Physics?

    Air resistance is usually taken into account via a differential equation; in simple situations, it grows linearly with the speed of the object.
  5. Nov 17, 2003 #4
    AP Physics is equivalent to first year college physics study.

    Thanks very much Jamesrc
    having some trouble figuring out the velocity (after the bat) but thanks for the help!
  6. Nov 17, 2003 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    There is no closed form solution for it.

    You need to multiply the force times a small step size and add it to the velocity and repeat. Works best on a computer.
  7. Jun 4, 2010 #6
    hey im doing an assignment for senoir school physics could you tell me how to calculate the air resistance on say the bonnet of a car?? nothing advanced just the basic formulae using the surface area of the bonnet and the force exerted by the air?? i know it has something to do with splitting it up into components??
  8. Jun 4, 2010 #7
    Look at


    The drag force depends on whether the air flow around the object is linear or turbulent, which then determines whether the drag force is proportional to the velocity, or the velocity squared. Also, the drag depends not only on the object size (frontal area) and shape but also on the surface roughness.

    Bob S
  9. Jun 5, 2010 #8


    User Avatar
    Homework Helper

    It turns out that in the real world it's more complicated, but instead of modifying the equation to one more realistic, Cd is redefined as a function (often implemented as an interpolated table) that varies with speed for a given medium, such as air, and a specific object (such as a bullet, http://en.wikipedia.org/wiki/External_ballistics).
  10. Jun 6, 2010 #9
    I was going to say this earlier, but I don't know the level of detail the OP needs.

    Jamesrc suggests taking an average value of Cd, in reality you cannot do this. As Cd depends heavily on Reynolds number. Above a critical number for a ball (and it depends on the ball) the Cd will be a steady 0.2sh. Below the critical Re number the Cd will raise until it nears the critical Re number and will then drop sharply.

    I did not mention it beucase if this is a generic textbook case for a baseball, his assumption of taking a middle Cd value is probably valid.
  11. May 9, 2011 #10
    hey guys i am yr 9 and doing a subject for my science fair this year: how much friction is redueced when a normal wheeled vechicle is changed into a maglev vechicle?
    how can i measure friction in this case
    and how can a measure air resistence
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook