Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Projectile motion

  1. Jan 24, 2010 #1


    User Avatar
    Gold Member

    1. The problem statement, all variables and given/known data
    Projectile A is launched horizontally at a
    speed of 20. meters per second from the top of
    a cliff and strikes a level surface below, 3.0 seconds
    later. Projectile B is launched horizontally
    from the same location at a speed of 30. meters
    per second.
    Approximately how high is the cliff?
    (1) 29 m (3) 60. m
    (2) 44 m (4) 104 m

    2. Relevant equations

    3. The attempt at a solution
    On two occasions my answer turn out to be '3' and '4'. but the answer key says it's '2'. I have tried for hours but i cant find the right approach. Please offer me a good explanation of how I should work with this? or u can even give me a link to read more or something.
  2. jcsd
  3. Jan 24, 2010 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    hi majormuss. If the projectile is launched horizontally, the horizontal component of velocity doesn't change (except by air friction which is ignored). Only the verticle component of velocity changes due to the acceleration of gravity. In this particular case, the initial verticle velocity is zero because it was launched horizontally. The verticle velocity then should be identical to a rock dropped from the same point. Can you figure out how far a rock would fall in 3 seconds?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook