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omnomnom

  • 3 years ago

A stone is dropped from the roof of a tall building. A person measures the speed of the stone to be 49 m/sec when it hits the ground. The height of the building is closest to: Select one: a. 24 meters. b. 49 meters. c. 122 meters. d. 245 meters.

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  1. Decart
    • 3 years ago
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    \[mgh = \frac{ 1 }{ 2 }mv ^{2}\]

  2. Decart
    • 3 years ago
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    you can subtract the mass from both sides which shows that everything falls at the same rate

  3. Decart
    • 3 years ago
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    you know what g is

  4. omnomnom
    • 3 years ago
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    Gravity?

  5. Decart
    • 3 years ago
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    yes

  6. Decart
    • 3 years ago
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    so if you plug in the velovity and solve for h which is the height

  7. Decart
    • 3 years ago
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    is this for conservation of energy

  8. omnomnom
    • 3 years ago
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    No this is only for the height of the building

  9. Decart
    • 3 years ago
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    The formula I gave you is the conservation of energy

  10. wio
    • 3 years ago
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    In this case \(g\) is the gravitational acceleration we talked about in the previous problem.

  11. Decart
    • 3 years ago
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    You cannot solve this with kinematics there are too many unknowns

  12. wio
    • 3 years ago
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    What do you mean you can't solve it with kinematics?

  13. Decart
    • 3 years ago
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    Yeah I guess you could solve for time in the acceleration then plug it in the average velocity.

  14. wio
    • 3 years ago
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    I was talking about the \(g\) in your formula.

  15. omnomnom
    • 3 years ago
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    ??

  16. wio
    • 3 years ago
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    Though technically speaking, depending on whether they are on the 'energy' part or the 'kinematic' part of the course decides which method they should use.

  17. Decart
    • 3 years ago
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    that is why I was asking

  18. wio
    • 3 years ago
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    Last question was a 'kinematic' one so maybe it must be kinematics.

  19. Decart
    • 3 years ago
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    acceleration of gravity = velocity final minus velocity inital divided by time.

  20. wio
    • 3 years ago
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    @omnomnom Have you talked about work or energy in your class yet?

  21. wio
    • 3 years ago
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    Like potential energy or kinetic energy?

  22. omnomnom
    • 3 years ago
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    No whats work?

  23. wio
    • 3 years ago
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    It is something you will learn in the future, most likely.

  24. Decart
    • 3 years ago
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    yes solve for time then plug into \[\frac{ d }{ t }=\frac{ v _{f}+v _{i} }{ 2 }\]

  25. Decart
    • 3 years ago
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    conservation is so much easier to solve

  26. omnomnom
    • 3 years ago
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    it confused me though the first equstion

  27. omnomnom
    • 3 years ago
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    equation*

  28. wio
    • 3 years ago
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    Don't worry about it.

  29. wio
    • 3 years ago
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    @omnomnom This is a two step problem. First you want to figure out how long it took to fall. Then you want to use that to figure out how far it fell.

  30. omnomnom
    • 3 years ago
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    oh okay so what equation do i use.... is is\[d =Vi( t)+ a \frac{ 1 }{ 2 }(t^2) \]

  31. wio
    • 3 years ago
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    You can't use that equation until you find time though.

  32. omnomnom
    • 3 years ago
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    Oh yeah >.<

  33. Decart
    • 3 years ago
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    \[a=\frac{ v _{f}-v _{i} }{ t }\]

  34. Decart
    • 3 years ago
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    you know a and both velocities

  35. omnomnom
    • 3 years ago
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    But we need the time :( Cant i just quess the answer?

  36. wio
    • 3 years ago
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    Yes, \(a\) is the same as last time... \(9.81m/s^2\)

  37. Decart
    • 3 years ago
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    velocity initial is 0

  38. wio
    • 3 years ago
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    You are not supposed to guess, because there is already a way to find the answer.

  39. omnomnom
    • 3 years ago
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    but...... Whats the way ?

  40. Decart
    • 3 years ago
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    t=49/9.81

  41. omnomnom
    • 3 years ago
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    answer is 5

  42. Decart
    • 3 years ago
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    yes 5 seconds

  43. Decart
    • 3 years ago
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    then plug in the time into your equation and find x

  44. omnomnom
    • 3 years ago
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    oh hold on

  45. omnomnom
    • 3 years ago
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    i got 49 m/s

  46. Decart
    • 3 years ago
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    that is the velocity

  47. omnomnom
    • 3 years ago
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    so we change the equation now?

  48. Decart
    • 3 years ago
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    you want the height of the building

  49. omnomnom
    • 3 years ago
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    yeah

  50. Decart
    • 3 years ago
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    x=1/2t(vfinal-vinital)

  51. omnomnom
    • 3 years ago
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    122.5 meters?

  52. Decart
    • 3 years ago
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    yes

  53. omnomnom
    • 3 years ago
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    thank you :D

  54. Decart
    • 3 years ago
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    do you understand the kinematic equations

  55. omnomnom
    • 3 years ago
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    The first one ? NO the others Yes

  56. Decart
    • 3 years ago
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    http://www.physicsclassroom.com/

  57. Decart
    • 3 years ago
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    this is a helpfull tool

  58. omnomnom
    • 3 years ago
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    oh kay thank you :D

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