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Homework Help: Some physics questions

  1. Sep 6, 2005 #1
    The answers that i think are correct have ***** next to them. If anyone knows the right answers please share your thoughts. Thank you.

    1. A ball is thrown downward (not dropped) from the top of a tower. After being released, its downward acceleration will be
    greater than g.
    exactly g. *****
    less than g.
    none of the above

    2. A rock is thrown up at a 45° angle. Neglecting air resistance, the x-component of the rock's velocity is
    greatest just after the rock is thrown
    greatest at the top of the rock's flight
    greatest just before the rock hits the ground
    always the same *****

    3. Velocity measurements
    differ in defferent frames of reference. *****
    are the same in any Galilean reference frame.
    are always largest in the reference frame of the Earth.
    are always smallest in the reference frame of the Earth.

    4. Which of these laws is not one of Newton's laws?
    Action force equals reaction force
    F = ma
    All objects fall with equal acceleration *****
    Objects at rests stay at rest, etc.

    5. Inertia
    expresses the tendency of bodies to maintain their state of motion. *****
    is Newton's third law.
    none of the above

    6. If vector B is added to vector A, the resultant vector A+B has a magnitude A+B when A and B are
    perpendicular to each other
    oriented in the same direction *****
    oriented in opposite directions
    in any direction relative to each other

    Thank you.
    Last edited: Sep 7, 2005
  2. jcsd
  3. Sep 6, 2005 #2
    I believe the only one you missed is #1. What is your reasoning for your choice? Think about it. When you throw a ball upward is there a change in velocity? What happens when you throw it down?
  4. Sep 7, 2005 #3
    Acceleration is only at a constant 9.8m/s2 in free fall only. So considering there is air resistance i guess that Acceleration has to be less than 9.8 m/s2 then. LMK
  5. Sep 7, 2005 #4


    User Avatar
    Science Advisor

    That argument I would accept. But you originally marked "none of the above"! Surely any acceleration must be "equal to g" OR "less than g" OR "larger than g"!
  6. Sep 7, 2005 #5
    how can acceleration be "larger than g"???
  7. Sep 7, 2005 #6
    so what would you say is the best answer from "equal to g" OR "less than g" OR "larger than g". I really don't think acceleration would be "larger than g" unless there is some kind of a trick to this question, but not really sure about the other 2 choices.
    Last edited: Sep 7, 2005
  8. Sep 7, 2005 #7
    G is the acceleration due to gravity? if yes, i think you could say that higher than G is possible... dont rely only on me, but id say that an object thrown downward has initial acceleration due to the hand which could be greater than G...
  9. Sep 7, 2005 #8

    Doc Al

    User Avatar

    Staff: Mentor

    The question is asking about the acceleration after being released, so any extra acceleration due to the force of the hand is irrelevant. The only ambiguity is whether to consider air resistance or not. Ignoring air resistance, the acceleration is exactly g. Air resistance reduces the acceleration (consider the net force on the object), since the object is thrown downward.

    (Halls' point was that the first three choices cover all the possibilities! :smile: )
  10. Sep 7, 2005 #9
    Thanks! I'll know it from now on
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