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: Velocity and Loss in Energy onedimensional

  1. Mar 22, 2012 #1
    1. The problem statement, all variables and given/known data
    If a 15kg object is released from a height of 11.58m and the objects loss in energy is 302 J, determine its velocity as it enters the container (object is being dropped straight down into a container)
    Assume g = 9.8m/s^2
    m(object) = 15kg = w(object) = 147N
    s = 11.58m
    ΔE = -320 J

    2. Relevant equations
    Had no idea. But the ones i know that i thought could apply:
    ΔKE = KE(final) - KE(initial)
    KE = 1/2 * mv^2
    GPE = mgh
    v^2 = u^2 + 2as

    3. The attempt at a solution
    Okay, I had no idea how to do this, but i tried to work out as many things as i can then mash em together to get an answer :p

    GPE = mgh

    Then i thought; well if the loss in energy is 320 J then i could take that from the original GPE
    1702.26 - 320 = 1382.26 J

    and now i could work out the new height by subbing it back in
    1382.26 = 15*9.8*h

    And here is where it gets iffy... they say the velocity when it enters the container, but the container isn't at 9.403m its at 0...right? thats what i got from the question anyway. Therefore how did they only lose 320N? So I assumed that I could say its KINETIC energy is the 1382.26 J, and the height of the container is the difference because they say when it "enters".
    KE = 1/2 * m * v^2
    1382.26 = 1/2 * 15 * v^2
    2764.52 = 15 * v^2
    v^2 = 184.30
    v = 13.58 m/s

    We don't have the answer to this question yet, but *a lot* of other people got 13.66m/s as the answer... so since i had no idea how to do this question and no basis to support whether im right or not, could someone help me out and either point out where i went wrong or if I'm right? thank you!
    Last edited: Mar 22, 2012
  2. jcsd
  3. Mar 22, 2012 #2
    I think they mean by lose of energy that it's due to friction, because when dropping an object from a certain hight the object does't lose energy but the energy is converted from potential to kinetic, and so when the question states that there there was loss in energy i think its due to friction(i might be wrong i don't know). If this is the case then i think you should do this:
    Energy (at h=11.58 )-Energy lost=1702.26-320=1382.26
    in this case you take the barrol at hight 0.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook