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Projectile motion ball thrown down vertically

  1. Feb 19, 2013 #1
    1. The problem statement, all variables and given/known data
    A ball is thrown vertically downward with a velocity of 30 ft from the top of a building.


    a.)What it's speed will be after falling 2 seconds
    b.)How far it will fall in 2 seconds
    c.)What will be its velocity after 30 ft
    d.)If the ball was released at 120 ft above the ground, in how many seconds will it strike the ground.

    2. Relevant equations

    For a I used the equation Vy = Vyo - gt
    For b I used the equation ΔY=Vyo(t) - 1/2gt^2
    For c I used the equation ΔY=Vyo(t) - 1/2gt^2 in order to find t, or the time it takes to move 30 ft and I got .72 s. Then I put that into the first equation of Vy=Vyo - gt.
    For d I used ΔY=Vyo(t) - 1/2gt^2.

    3. The attempt at a solution

    For a I got -94 ft/s.
    For b I got -124 ft/s.
    For c I got 53.04 ft/s.
    For d I got 2.74 seconds

    I have a few questions. First, can someone check if these answers are correct? My professor did not provide an answer sheet for the exam review. Also I was only able to get a proper answer for d if I made the 120 ft negative. If the ball was released at 120 ft above the ground wouldn't it be positive? Also, c gave me some trouble, if it is incorrect, could someone please provide an in-depth explanation? Thank you so much.
  2. jcsd
  3. Feb 19, 2013 #2
    I think your answers for (a) and (b) are correct.

    For (c), it should be -53.04ft/s.
    Instead of working out the time first, you can just use 1 equation :
    V2 = V02 - 2gΔy

    For (d), you release the ball at the origin of your coordinate system, so the ground is at y=-120ft.
    From (b) you know the ball falls 124ft in 2s, so to fall 120ft will be just under 2s, so your answer is not correct here.
  4. Feb 19, 2013 #3


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    Staff: Mentor

    You should be careful with the units - "30ft" is not a velocity, and "-124ft/s" is not a length.
  5. Feb 19, 2013 #4
    Well the question for d did not specify an initial velocity, it just said released, so I think it's right.

    Oh right, I better not make that mistake on the exam haha.
  6. Feb 20, 2013 #5
    For part (d), yes you're correct.
    I didn't notice it started from rest.
  7. Feb 20, 2013 #6
    Cool, thank you very much for your assistance.
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