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Falling objects: Tennis ball vs. feather

  1. Jul 27, 2015 #1
    Assuming air resistance, when a tennis ball and a feather are dropped from the same height? Which statement is true:

    A: Tennis ball has more mass so it will fall faster.
    B: Tennis ball has less surface area so it will fall faster

    I think the answer should be A, but the textbook say B is true base on the reasoning that surface area is proportional to air resistance. My arguments are as follows:

    1. The question does not suggest a feather have a greater surface area than a tennis ball. I can picture the size of a tennis ball, but size and shape of feather varies depending on the source.

    2. Assuming if they explicitly state that cross-sectional surface area is greater for the feather, wouldn't the mass still play a greater effect due to the greater difference in mass between the objects vs. the difference in cross-sectional areas.

    Thank you.
     
  2. jcsd
  3. Jul 27, 2015 #2

    Nugatory

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    Neither A nor B are correct; if your book says B either it's wrong or you're misunderstanding what it's saying. What matters is the ratio of mass to surface area, and that is greater for the tennis ball.
     
  4. Jul 27, 2015 #3
    But in this case, wouldn't the mass is the main factor in contributing to the difference in ratio? So wouldn't A be "more" correct than B given those two choice.

    Here is the full solution in the book:
    The force of air resistance is dependent on the surface area, the shape, and the velocity of an object; the object's mass does not affect it. Surface area is directly proportional to air resistance. The feather has a larger surface area and experience greater resistance, or upward force. This results in smaller net force and smaller acceleration downward. Acceleration due to gravity is the same for all objects near earth's surface regardless of mass.

    Edits:
    Here a link I found that kinda explain why it is the mass that play a bigger role:
    http://www.physicsclassroom.com/mmedia/newtlaws/efar.cfm
     
    Last edited: Jul 27, 2015
  5. Jul 28, 2015 #4

    Drakkith

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    No. It just happens to be the case that the tennis ball is much more massive than the feather, but I can easily imagine an object with the same mass as the tennis ball yet a hundred times its surface area. The surface area is just as important as the mass of the object. Neither one has a greater effect than the other unless you directly specify some arbitrary criteria.

    In my opinion, you should probably burn this book and never look at it again. This is a ludicrous explanation. As Nugatory explained, it's the ratio of the mass to the surface area that determines the acceleration through the air. This comes about because the downward force of gravity increases with mass while the force from air resistance increases with surface area. I have no idea why they are even bringing in the acceleration of gravity as part of the explanation. You aren't even using the acceleration under gravity, but the force.

    I cannot stand these 'cause and effect' style question. They are absurdly easy to mislead students with. For one, there usually isn't a single cause, as the question in your example shows. Both A and B are potentially true.
     
  6. Jul 28, 2015 #5

    A.T.

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    There are feathers smaller than a tennis ball, but still fall slower.
     
  7. Jul 28, 2015 #6

    russ_watters

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    But what was the exact question?
     
  8. Jul 28, 2015 #7
    Go back to basics, if you're on the moon which would drop faster, the feather or the tennis ball, Google hammer and feather on the moon, there must be a you tube video out there. Having seen what happens on the moon think about what would happen on Earth.
     
  9. Jul 28, 2015 #8

    PeterDonis

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    The moon has no air, so no air resistance. The OP explicitly said "assuming air resistance".
     
  10. Jul 28, 2015 #9

    PeterDonis

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    To see the flaw in this argument (which is basically that the mass of the object doesn't matter), consider a tennis ball and an oversized ping-pong ball that's the same size as the tennis ball (but obviously much less massive). If surface area were the only factor that mattered, both of these would fall at the same rate. Do they?

    Or, to be even more extreme, consider a ping-pong ball and a ball of lead of the same size.

    In fact, you could do the experiment with an actual tennis ball and an actual ping pong ball; if smaller surface area means falling faster, the ping pong ball should fall faster than the tennis ball. Does it?
     
  11. Jul 28, 2015 #10
    Yes it does, but by considering the case without air resistance you can dismiss answer A, and then unpick the bad wording of answer B.
     
  12. Jul 28, 2015 #11

    A.T.

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    ...you fail to answer the question.
     
  13. Jul 28, 2015 #12
    A rocket can have a large surface and still meet relatively little resistance.
    Burn the book.
     
  14. Jul 28, 2015 #13
    Or, alternatively, you could just get a couple of feathers of different sizes and a tennis ball and drop them about 7 or 8 feet off the ground, inside with no wind, and just watch. My incredibly intuitive prediction: Tennis ball 1, feather 0. Tennis ball gets down faster. For one thing, the feather won't even fall straight down, it will wiggle from side to side on its way down so it will travel more distance than the ball which will fall more or less straight down. THEN throw out the book.

    I assume you know you drop the same objects on the moon with no air they hit the ground at the same time, no matter how high they are dropped.
     
  15. Jul 28, 2015 #14

    PeterDonis

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    No you can't. The mass of an object does affect how fast it falls when air resistance is present, even though it doesn't when air resistance is not present.
     
  16. Jul 28, 2015 #15
    >>The force of air resistance is dependent on the surface area, the shape, and the velocity of an object; the object's mass does not affect it.

    Note it is referring to the FORCE of air resistance. The actually change in the velocity of this object, because of this force, will depend upon the mass of the object. ( F=ma )
    So I'd have to say this text is technically correct.
     
  17. Jul 28, 2015 #16

    A.T.

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    "Surface area is directly proportional to air resistance. The feather has a larger surface area and experience greater resistance, or upward force. This results in smaller net force..."

    This assumes that all other forces (like weight) are the same. Otherwise you know nothing about the net force.
     
  18. Jul 28, 2015 #17
    >>This assumes that all other forces (like weight) are the same. Otherwise you know nothing about the net force.

    I don't see why that statement needs to make any such assumption. I agree that to compute the actual NET force you will need to know the weight. However, it is true to state that, regardless of the weight, the greater the air resistance upward, the less the NET force downward. Fnet = Fweight + -Fairresistance

    Don't get me wrong, the initial question from the textbook is awful. The only way it makes sense is if you use the statement-part of the answers, as information available to answer the question: "Tennis ball has more mass", and "Tennis ball has less surface area"
     
  19. Jul 28, 2015 #18

    A.T.

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    No, you cannot say that the net force is less, without making any assumptions about weight.
     
  20. Jul 28, 2015 #19
    The question is about which go faster so exactly about the change in velocity you mention. It is not about comparing forces of resistance but velocities
     
  21. Jul 28, 2015 #20
    You are right; I was assuming weight stayed the same, and air resistance changed!

    Still, I don't see why the book statement I vouched for needed to make that assumption, it doesn't even reference the net force:
    >>The force of air resistance is dependent on the surface area, the shape, and the velocity of an object; the object's mass does not affect it.
     
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