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Question on simple acceleration concept

  1. Jul 5, 2009 #1
    1. The problem statement, all variables and given/known data

    A ball is thrown straight up into the air. If we do not ignore air resistance, the acceleration of the ball as it is traveling upward is....?

    a.) 9.8m/s^2
    b.) greater than 9.8m/s^2
    c.) less than 9.8m/s^2
    d.) zero


    2. Relevant equations

    n/a

    3. The attempt at a solution

    My line of thinking is that if there were air resistance, then the ball would travel slower. Therefore I thought the answer would be less than 9.8m/s^2. Correct answer is greater than 9.8m/s^2. Can somebody explain? Thanks.
     
  2. jcsd
  3. Jul 5, 2009 #2

    tiny-tim

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    Welcome to PF!

    Hi test2morrow! Welcome to PF! :smile:
    Hint: which direction is the acceleration? :wink:
     
  4. Jul 5, 2009 #3
    This is a very misleading question. If gravity pulls downward, but we define upward as the positive direction (as would most people), then acceleration is negative, not positive. In that case the answer would still be "less than" because acceleration is negative and only positive values are given. But it might also be reasonable to assume the author of the question was really talking about only the magnitude of the acceleration OR thinking of upward as negative, because only positive accelerations are given (even though the author doesn't mention it). So in that case the air adds to the force of gravity as the ball goes up (but subtracts as the ball goes down), so the answer would be "greater than". Since the question is ambiguous and the optional answers imply something that only some students might presume, I say it is a bad question.
     
  5. Jul 5, 2009 #4
    when its going up 2 forces work in the same direction (air drag and gravity) so the net force is ____ ,when its going down gravity works downwards and air drag upwards thus net force is_____ , when the net force is larger the acceleration is larger (in size)
    can u fill the holes?
     
  6. Jul 6, 2009 #5

    turin

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    "less than" and "greater than" don't have an unambiguous vectorial meaning, and magnitude is most certainly implied. It is not a bad question; it is a very good question as demonstrated by test2morrow's (commonly made) mistaken reasoning. One must distinguish speed and acceleration. IMO, the best way to reason this problem is to simply draw the fbd. Then, you will see that both force vectors point in the same direction, resulting in an acceleration (from Newton's second law) that is greater than the acceleration of gravity alone.
     
  7. Jul 6, 2009 #6
    So which do you think is the right answer?
     
  8. Jul 6, 2009 #7

    turin

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    b) greater than 9.8 m/s^2, of course.
     
  9. Jul 6, 2009 #8
    Wrong. A positive acceleration will cause the upward moving ball to increase its upward speed, and you are saying air resistance increases it even further. The acceleration of the Earth is downward (and thus negative), is it not? My point here is the question rewards imprecise thinking and punishes precise thinking. Or shall we always consider upward as negative and downward as positive when we have a choice? Most mistakes involve imprecise thinking.
     
  10. Jul 6, 2009 #9

    turin

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    No law of physics says that I must choose down to be negative. You can think of it this way: I chose down to be positive in this problem.
     
  11. Jul 6, 2009 #10
    And I chose up.

    Still think its a "good" question?
     
  12. Jul 6, 2009 #11

    turin

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    Yes, and I'm done arguing with you.
     
  13. Jul 6, 2009 #12

    tiny-tim

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    I agree :smile: … it obviously means magnitude …

    there's no other sensible way of reading the question. :rolleyes:
     
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