1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
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: Maximum velocity of object dropped from cliff

  1. Apr 10, 2013 #1
    Just finished an exam. One problem is bothering me and I want to be certain if it's right.

    1. The problem statement, all variables and given/known data
    What is the maximum velocity of an object dropped from a 20 m cliff?
    Assume no air friction and gravity is 10 m/s/s.

    2. Relevant equations
    None given, but I assume v=gt is relevant.

    3. The attempt at a solution
    15 m/s.
    My reasoning was that if after 1s, the object fell 10m at 10m/s,
    and after 2s, the object has fallen 30m (10m + 20m) at 20m/s (10m/s + 10m/s),
    then I can divide the 2nd speed increment in half to determine speed at 20m fall (10m/s + 5m/s).

    I tried using equations before answering 15.
    v=gt by itself can't work without t (t was not given).
    d=1/2gt^2 tells me the object fell for 2s, but that doesn't seem right (it would have traveled 30m).
    Assuming t=2s is right, v = 10m/s/s * 2s = 20m/s.

    So which is right: 15 or 20?
  2. jcsd
  3. Apr 10, 2013 #2


    User Avatar
    Homework Helper

    It's 20.
    In the first second it reaches 10m/s, but travels only 5m [average speed = (0+10)/2 = 5
    In the second second second is increases to 20 m/s, travelling a further 15m [average speed = (10+20)/2 = 15.
  4. Apr 10, 2013 #3
    Oh, that's right... it has to be averaged since speed isn't constant. Thanks.
  5. Apr 11, 2013 #4


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    In my day we would remember one of the the SUVAT Equations


    probably V2 = U2 + 2aS


    V = final velocity
    U = initial velocity
    a = g ≈10
    s = displacement

    Reduces to

    V2 = 2as
    = 2 * 10 * 20
    = 400

    so V=20
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted