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How does weight affect air resistance?

  1. Nov 11, 2014 #1
    How does weight affect air resistance? [i.e A heavy feather falling v.s a light one}Please be simple
     
  2. jcsd
  3. Nov 11, 2014 #2
    Weight has no effect at all on air resistance. (Is that sufficiently simple?)
     
  4. Nov 11, 2014 #3

    Bandersnatch

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    I would think that despite the wording the OP actually wants to know how weight and air resistance affect the rate of falling.

    In the suggested case of two feathers of different mass - let's imagine one is an actual feather, the other is an iron-cast copy - the gravitational force on the iron one is much higher than that on the organic one (gravitational force scales with mass). However, as mentioned in the post above, the air resistance is not affected by mass (instead it's affected by shape, size, air density and object velocity) so as a result the net force pulling a feather downwards will be reduced through air resistance by a smaller fraction of the weight in the case of a heavier feather, resulting in higher acceleration and a quicker fall.
     
  5. Nov 11, 2014 #4
    Mass hasn't a "direct" effect, but it plays an important role becouse of the kinetic energy. We can take as exemple a truck and a small car: even if they travel at the same speed the truck is harder to stop and this difference doesn't depend on the brake (in your question air resistance), but on the "quantity of moving stuff" (more precisely, the momentum)
     
  6. Nov 11, 2014 #5
    continuing on what bandersnatch said if the both the feather and iron cast was on the moon where there is no gravity both would fall exactly. The main factor is air resistance weight does not affect air resistance.
     
  7. Nov 11, 2014 #6

    Bandersnatch

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    I think you meant to say "no air". ;)
    Welcome to PF, by the way.
     
  8. Nov 11, 2014 #7

    OldEngr63

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    Guess what!! There is gravity on the moon also, even though it is less than that on the earth. Further, what exactly does it mean to say, "... both would fall exactly"? The both fall on earth, on the moon, and anywhere else that there is a gravitational attraction.

    The original question was about weight, not mass (at least that is the way I read it). It was also about air resistance, not energy.
     
  9. Nov 12, 2014 #8

    CWatters

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    No. In general weight does not affect air resistance. Air resistance depends on shape. If both feathers are the same shape their drag properties ("air resistance") will be the same. In that case they will both fall at the same rate. Light and heavy objects fall at the same rate for the following reason... The force due to gravity is proportional to mass but at the same time the acceleration is inversely proportional to mass. So the net result is no difference.

    In some cases the distribution of the mass might make a difference to the air resistance. For example by changing which way up the object falls. If you had two identical size and shape feathers but one had a different mass distribution one might fall "point down" and the other "flat". That would make the air resistance different.
     
  10. Nov 12, 2014 #9

    jbriggs444

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    A feather from a bird and an identically shaped feather crafted from lead will have identical air resistance but different weight. The net force on them (air resistance plus weight) will NOT be proportional to mass. This means that they will NOT fall at the same rate.

    In the special case where air resistance is zero the net force is proportional to mass and the feathers fall at identical rates.
     
  11. Nov 12, 2014 #10

    OldEngr63

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    Wasn't this resolved experimentally several hundred years ago?
     
  12. Nov 12, 2014 #11

    PeroK

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    I know what you mean, but I suspect rigidity and suppleness might have an effect on air resistance as well as shape!
     
  13. Nov 12, 2014 #12

    Nugatory

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    Suppleness and lack of rigidity allows the forces of air resistance to deform the object and change the shape, which in turn affects the forces. So yes, your intuition that they matter is good... and this is one of the reasons that highly accurate calculations of air resistance for real objects is such a hairy problem.

    However, for any reasonably rigid object, the effect of deformation under aerodynamic stress is well and thoroughly negligible.
     
    Last edited: Nov 12, 2014
  14. Nov 12, 2014 #13

    sophiecentaur

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    Some messages take a while to get through, though. People would often rather trust their intuition than the logic behind a Physical explanation.
     
  15. Dec 18, 2015 #14
    Drag force = 0.5*(density of fluid)*(velocity^2)*(drag coefficient)*(cross sectional area) so weight does not affect the drag force. However, with everything else staying the same (shape/cross sectional area), the weight will determine how quickly terminal velocity is reached as seen in the following equation:

    a = F/m = (W - D)/m Once the drag (D) equals the weight (W), you reach terminal velocity i.e. you do not accelerate anymore and velocity is constant.
     
  16. Dec 20, 2015 #15

    David Lewis

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    The heavier of two identically-shaped falling objects often has a lower drag coefficient. As Reynolds number goes up, drag coefficient usually goes down.
     
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