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Homework Help: MCAT kinematics problem. Please help!

  1. Nov 22, 2011 #1
    1. The problem statement, all variables and given/known data

    A penny with mass of 3.1 grams is dropped from skyscrapper of height 1250m. Find the objects acceleration after 5 secs. If a penny with mass 2.5g is dropped at this point how far does it travel before the first penny hits the ground?

    2. Relevant equations
    Kinematic equations; f=ma.
    Last edited: Nov 22, 2011
  2. jcsd
  3. Nov 22, 2011 #2
    Is there any additionally information for the problem? The question is stated in such a manner that it leads me to believe there should be some air resistance. If there is no air resistance, the penny will accelerate at a constant rate of 9.81N/kg.
  4. Nov 23, 2011 #3
    Remember Galileo's experiment (assuming no air or other resistance).

    Two objects dropped from the same distance will fall for the same amount of time regardless of its mass.

    So how long does it take penny A to hit the ground?

    After you figure that out, how long is penny B falling before penny A hits the ground?
    (hint: penny B is dropped 5s after penny A)

    After you figure that out, how far does penny B fall in that amount of time?

    Hope this helps!
  5. Nov 23, 2011 #4
    @ Sandy.bridge - yes that is all the information given. So assuming air resistance is negligible the first penny has an accceleration of 9.81N/kg??

    What about the second penny?>>> If a penny with mass 2.5g is dropped at this point how far does it travel before the first penny hits the ground?
  6. Nov 23, 2011 #5
    Take note of the remark LabGuy330 stated.
  7. Nov 23, 2011 #6
    Thanks labguy!

    But how can I find how long it takes penny A to hit the ground if I dont have value for v?
    Can I use. d= vit + 1/2 at^2 and make vi=0?
  8. Nov 23, 2011 #7
    You do have a value for v (actually vi). It is zero! That is, it starts at rest.
  9. Nov 23, 2011 #8
    Thanks all! I worked it out using d= vit + 1/2 at^2.
    -Found time it took penny A to reach hit ground ~15 secs.
    -Time for penny B is 15 + 5 secs= 20s
    -Then using d= vit + 1/2 at^2 found how far penny B lands. 2000m.
    Can someone please verify if this is correct??
  10. Nov 23, 2011 #9
    It seems your rounding is way off ... the time for penny A is closer to 16 secs.
    Also, the problem isn't asking how long it take penny B to hit the ground (it would take the same amount of time as penny A). Rather, it is asking how far penny B falls before penny A hits the ground. (Besides, it can't fall more than 1250 m, so 2000 m can't be right)
  11. Nov 23, 2011 #10
    I tried labguys approach by finding the time and such. Should I go about it differently? If so how?
  12. Nov 23, 2011 #11
    1) Determine the time it takes for Penny A to reach the ground from the time it is dropped
    2) 5 seconds later Penny B is dropped, so find out how much time is left for Penny A to reach the ground
    3) How far does Penny B travel in that time?
  13. Nov 23, 2011 #12
    just a hint : the second penny travels for 16-5 seconds...
    do the math now !!
  14. Nov 23, 2011 #13
    Penny B travels 605 m . Got it! thanks everyone!!
    Last edited: Nov 23, 2011
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