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Homework Help: I don't understand this question could someone clarify?

  1. Nov 4, 2009 #1
    1. The problem statement, all variables and given/known data
    determine the frictional force of air on a body of mass 0.25 kg falling with an acceleration of 9.2 m/s^2

    2. Relevant equations

    I don't know?

    3. The attempt at a solution

    I don't even know what they are asking.. this is on the newton's laws chapter.. so what is a "frictional force of air"? I'm guessing that it is the opposite vector to the air then? by the 3rd law? so maybe something like W+Airforce = 0.25(9.2) and then you can solve for the airforce and then use the 3rd law to say that the vector in the opposite direction is opposite?

    thank you
  2. jcsd
  3. Nov 4, 2009 #2


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    What would the acceleration of the falling object be if there were no friction?
  4. Nov 4, 2009 #3
    thank you for the reply. well, we are given the acceleration right? 9.2 m/s^2? am I right in saying that I am supposed to find the opposite "reaction" vector to the force of the thing falling down? thank you!
  5. Nov 4, 2009 #4


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    If gravity were the only force acting on the mass, the acceleration would be 9.8 m/s2. The acceleration is not that. How much force opposite to gravity is required to reduce the acceleration to 9.2 m/s2?
  6. Nov 4, 2009 #5
    Ohhh!!! I see!! wow, this is a cool question. so I guess since the falling acceleration is less than gravity (the standard falling acceleration?) then something is acting to slow it down.. that is the "frictional force of air"? but gravity isn't a force, so I have to consider the
    weight-F = 0.25(9.2) ?

    also, on the side.. if the acceleration were greater than 9.8, that would mean something is acting to make to go faster right? so I would do weight + F = 0.25(faster acceleration)?

    thanks :) I used to hate physics but it is starting to look very cool
  7. Nov 4, 2009 #6
    That is exactly right.
  8. Nov 4, 2009 #7
    perfect! thank you!
  9. Nov 4, 2009 #8
    Hey there;

    well this is a very usual example but you can always think about raindrops when it comes to friction of air;

    raindrops falls with a safe velocity, means that they dont harm you when they fall to your head.If there was no "air friction" a raindrop that is falling to the ground from a cloud which has an altitude of 1km lets say, should have a velocity about 507.6 km/hr.Well a raindrop with that velocity will kill you instantly....
  10. Nov 4, 2009 #9


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    Not so fast. The speed of the raindrop is 140 m/s. Assuming a radius of 3 mm, the mass of the drop is 1.13x10-4 kg which gives it a momentum of 1.6x10-2 kg*m/s. A 0.040 kg (40 gram) bullet that has the same momentum, will have to be traveling at 1.6x10-2/0.04 = 0.40 m/s which is a slow walking speed. You can certainly stop that bullet without being Superman and therefore you can stop the drop too.
  11. Nov 4, 2009 #10
    Imagine a heavy rain then :D, well to be honest i didnt calculated the momentum for a raindrop, but a hail would surely be deadly without air friction. :)
  12. Nov 4, 2009 #11
    interesting remarks, thank you.... I thought that air friction would actually have something to do with the friction caused by air... what is air resistance then? I wish they would be more clear about what they want :(
  13. Nov 4, 2009 #12
    You're one of us now muh-ha-ha-ha-ha! stirthepot.gif
  14. Nov 4, 2009 #13
    Air friction is the force caused solely by friction between the object and the air. Air resistance is the total dragging force of the object caused by the air, it includes both the frictional force and the force caused by the object pushing the air out of the way, which is determined by the shape of the object and the speed of the object.
    Last edited: Nov 4, 2009
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