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Projectile Problem: Marble falls from Desk

  1. Feb 25, 2017 #1
    Hey Guys, I have attempted the question below and need someone to check my work, please.

    Thanks!!

    1. The problem statement, all variables and given/known data

    A marble rolls off a desk that is 0.86 m above the floor with a horizontal speed of 1.2 m/s.

    a) What is the marble's horizontal velocity when it is half way to the floor?
    b) What is the marble's vertical velocity at that point?
    c) Describe the value of the vertical component of velocity as the marble falls to the floor.​

    2. Relevant equations


    3. The attempt at a solution
    20170224_173634[545].jpg
     
  2. jcsd
  3. Feb 25, 2017 #2

    PeroK

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    For c), I don't think that ##0## is the answer they are looking for!
     
  4. Feb 25, 2017 #3

    PeroK

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    Also, if you are using ##g## to one decimal place, you cannot have an answer for velocity to four decimal places.
     
  5. Feb 25, 2017 #4
    Will it be 2.9008 m/s?
     
  6. Feb 25, 2017 #5
    It is not correct to write ##\vec g = - 9.8 m/s^2## , because a vector is never equal to a number.

    After the ball leaves the table, the vertical component of its speed increases from ##9.8 m/s## to every second until it collides with the ground.
     
    Last edited: Feb 25, 2017
  7. Feb 25, 2017 #6

    PeroK

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    No, you just do what you did to find its velocity half way to the floor.

    You can safely ignore post #5.
     
  8. Feb 25, 2017 #7
    I get it, thank you.
    It will be 4.1 m/s.
     
  9. Feb 25, 2017 #8

    PeroK

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    That looks about right.
     
  10. Feb 25, 2017 #9
    In letter c he does not ask for speed by hitting the ground. He asks what happens to the speed over the fall.
     
  11. Feb 25, 2017 #10

    haruspex

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    Good point.
     
  12. Feb 25, 2017 #11
    For this, see post # 5 "ignored".
     
  13. Feb 25, 2017 #12

    PeroK

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    It looks like they weren't asking for the speed as it hits the floor, but a "description of the value" during the fall.
     
  14. Feb 25, 2017 #13
    Exactly
     
  15. Feb 25, 2017 #14

    PeroK

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    The thing to ignore was about the vector. The OP's diagram is good and clear in its vector representation, in my opinion.
     
  16. Feb 25, 2017 #15
    The problem is that he wrote ##\vec g= - 9.8 m/s^2##
    The correct is ##\vec g= -9.8 \vec j m/s^2## (vector equation) or else I could write ##g=-9.8 m/s^2## (scalar equation)
     
  17. Feb 25, 2017 #16

    PeroK

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    The OP's diagram is clear and that's just pointless pedantry.
     
  18. Feb 25, 2017 #17
    When writing this it is being said that a vector is equal to a number. A vector can only be a vector and a number can only equal a number.
     
  19. Feb 25, 2017 #18
    Ok.
     
  20. Feb 25, 2017 #19

    haruspex

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    Yes, you've made your point, but it really is a bit excessive to complain about that in an explanatory note in a diagram.
     
  21. Feb 25, 2017 #20
    I agree. It's just a matter of formality.
     
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