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This is about MOMENTUM (Practice Test #3 Question #1)

  1. Oct 8, 2014 #1
    1. The problem statement, all variables and given/known data
    . A rock is dropped from a high tower and falls freely under the influence of gravity. Which one of the following statements concerning the rock as it falls is true? Neglect the effects of air resistance.


    (a) The rock will gain an equal amount of momentum during each second.

    (b) The rock will gain an equal amount of kinetic energy during each second.

    (c) The rock will gain an equal amount of speed for each meter through which it falls.

    (d) The rock will gain an equal amount of momentum for each meter through which it falls.

    (e) The amount of momentum the rock gains will be proportional to the amount of potential energy

    that it loses.


    2. Relevant equations
    gravity which is equal to 9.8 m/s^2 => I am using 10 m/s^2 for simplicity


    3. The attempt at a solution
    (A) yes, its possible
    (B) no, its not possible
    (C) question:

    If I have a rock and i throw it from the second floor (4 meters from the ground), wouldn't the rock gain the same speed as it goes down each meter??? (lets assume is 10m/s^2 instead of 9.8 m/s ^2)

    1 meter down and 1 second passes = 10 m/s
    2 meter down and 2nd second passes = 20 m/s
    3 meter down and 3rd second passes = 30 ms

    so what I can see is that each second and each meter , the rock gains the same speed. So, it gains 10 m/s each meter and second down
     
  2. jcsd
  3. Oct 8, 2014 #2

    jedishrfu

    Staff: Mentor

    For question c, I think you're wrong when you equate meters to seconds.

    Remember that the distance fallen is s=1/2 g t^2 so in the first second its fallen 1/2g or 4.9m and in the second second its fallen is 2 g or 19.6m.
     
  4. Oct 8, 2014 #3

    Simon Bridge

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    (c) is asking about each meter the rock falls, not about each second.
    Does the rock take the same amount of time to fall each meter?
     
  5. Oct 9, 2014 #4

    Yes??? doesn't the rock take 1 second to reach to the next meter? a meter per second?? so it takes the same amount (that is the way i understand it) or DO THE SECONDS INCREASE????
     
  6. Oct 9, 2014 #5

    UH? sorry to difficult for me to understand. S = 1/2 g t^2? what is that?
     
  7. Oct 9, 2014 #6

    Simon Bridge

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    If the rock takes 1s to reach the next meter all the time, then the speed must be constant.
    But doesn't the speed of a falling object increase with time?
    In which case, doesn;t that mean it falls further with each second?
     
  8. Oct 9, 2014 #7

    yes, it does increase with time as it is accelerating right? so the time doubles or what?
     
  9. Oct 9, 2014 #8
    so if the rocks is accelerating then no way can the speed be the same.

    so C is wrong
     
  10. Oct 9, 2014 #9
    Can you tell me who a rock falls with a graph. I had this idea before

    10 meter for the 1st second
    20 meters for the 2nd second
    30 meters for the 3rd second

    something isn't right about this
     
  11. Oct 9, 2014 #10

    Simon Bridge

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    The equation you can get from the suvat equations.
    ##d=\frac{1}{2}gt^2##
    ... distance d fallen at the end of t seconds.
     
  12. Oct 9, 2014 #11
    thnks
     
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