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ARGH. Multiple multiple choice

  1. Sep 24, 2004 #1

    Alkatran

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    2. [2pt] Pick the statement which correctly describes the acceleration, a, of the objects described below (neglecting air resistance). Select from
    A: a=0 m/s^2
    B: 9.81 m/s^2 down
    C: a=9.81 m/s^2 up
    D: a>9.81 m/s^2 down
    E: a<9.81 m/s^2 down

    F: a>9.81 m/s^2 up
    G: a<9.81 m/s^2 up
    H: Not enough information
    E.g., if the first answer is A and the rest C, enter ACCCCC.

    1) Just after a bullet is shot upward from a gun
    2) An elevator moving downward at constant speed
    3) At the top of the path of an object thrown upward
    4) While a ball is in a person's hand during a downward throw
    5) An elevator starting on its descent
    6) While a ball is in a person's hand during an upward throw


    problem 1: the italicized letters could be taken as less than -9.8 or greater than -9.8 for both! (is it less than 9.8 down, as in 5 down is less than 9.8 down or -10 is less than -9.8???)

    1,2 and 3 are obviously B, 4 is D. the problem is 5 and 6
    5 could be: E, D, G, H (or in extreme cases B)
    6 could be: F, G, C, H

    Combinations I've tried so far:
    BBBDBC
    *they all start with BBBD, so I'll just put 5 then 6
    BF

    EB
    EC
    EE
    EF
    EG
    EH

    HB
    HH


    Help... please :surprised
     
  2. jcsd
  3. Sep 24, 2004 #2
    When they say "down" or "up" I believe that means that a>9.8 would be a magnitude and down/up specifies the direction. That's just what I got out of it.

    5. In the elevator, are you leaving the ground or do you just feel less weighty?

    6. The ball is accelerating upward. What does that tell you about the magnitude of the acceleration of the ball?

    Edit: How is an elevator moving at a constant speed accelerating at 9.8m/s^2?

    Double Edit: Ok I'm starting to think the questions are too vague. Resultant acceleration would say an elevator moving at a constant speed isn't accelerating but the tension in the cable is pulling upward with an acceleration of 9.8m/s^2 to achieve the constant speed.
     
    Last edited by a moderator: Sep 24, 2004
  4. Sep 24, 2004 #3

    Alkatran

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    An elevator beginning to move downward will have an acceleration less than 0. But it won't be more than 9.8 because then the people inside would have trouble keeping their footing. But this satisfies two conditions, < 9.8 down and > 9.8 up. It's pretty much the same scenario for the other question.
     
  5. Sep 24, 2004 #4
    If the elevator is accelerating downward it can't be accelerating upward. That would be crazy. The question could have been worded better if it was made clear that the accelerations were with respect to earths gravitational field. I agree with your first condition for # 5 and second condition for #6.
     
  6. Sep 24, 2004 #5

    Alkatran

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    Sorry, I meant < for both cases. (#5)
     
  7. Sep 24, 2004 #6
    I figured as much. Your first condition is right because yes the magnitude of the acceleration is less than 9.81m/s^2 in the DOWN (-1) direction so the acceleration is -9.81<a<0 with respect to the earth. Here's how I determine each condition with respect to the earth(Edit):

    a>9.81 down -9.81 < a
    a<9.81 down -9.81 > a > 0
    a>9.81 up 9.81 < a
    a<9.81 up 0<a<9.81

    Yay getting bogged down in semantics.
     
    Last edited by a moderator: Sep 24, 2004
  8. Sep 29, 2004 #7

    Alkatran

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    Haha, I figured out what one of the problems was.

    "An elevator moving downward at constant speed" is A, not B.

    But the last 2 still evade me. It stil won't take HH or EH or...
     
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