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Homework Help: Momentum problems

  1. Jun 1, 2014 #1
    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 will gain an equal amount of momentum during each second
    (b) The rock will gain and equal amount of kinetic energy during each second
    (c) The rock will gain and equal amount of speed for each meter through which it falls
    (d) The rock will gain and 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 is loses

    Can you explain me why A is the answer?

    does the rock gain an egual amount of mass and velocity as it falls? Lets say the rock weights a pound.

    1st second = 1 lb x 9.8 m/s
    2nd second = 1 lb x 9.8 m/s
    3rd second...

    so by the second second the rocks gain an extra pound?? that is confusing to me
    Last edited: Jun 1, 2014
  2. jcsd
  3. Jun 1, 2014 #2


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    Would the speed at the second still be 9.8 m/s?
  4. Jun 1, 2014 #3
    Does it make any sense to you that an object would gain mass as it falls?

    If so, get an eraser (or another not easily breakable object) and drop it. After reaching the floor/table has its mass increased?
  5. Jun 1, 2014 #4
    @Mafagato, of course the mass does not increase but why is the answer a correct?
    isn't P=mv ? so basically the answer is saying , yes, (m.v) increases every second
  6. Jun 1, 2014 #5
    @nathanel,yes, the speed increases every second at the rate of 9.8
  7. Jun 1, 2014 #6


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    Can you write the equation for the rock's velocity, v(t), at time t? Then P(t) = mv(t).
  8. Jun 1, 2014 #7
    So, you solved it, didn't you?

    9.8m/s. If the speed increases, p = m * v increases, right?
  9. Jun 1, 2014 #8
    @nafagato, of course it does, thanks!! :)
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