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: Simple mechanical energy question [gr.11]

  1. Mar 22, 2004 #1
    [urgent] simple mechanical energy question [gr.11]

    Several children, pretending they are playing in the jungle, suspend a rope from an overhead tree limb. A child of mass 40 kg running at 8.0m/s grabs the rope and swings off the level ground.

    a) What maximum height does the child reach?
    b) How fast would a 30 kg child have to run to reach the same height as the 40 kg child?

    ½mv1²+mgh1 = ½mv2²+mgh2

    a) m=40kg, v1=8.0m/s, h1=0m, g=9.8m/s², v2=0m/s (at max height v=0)


    ½mv1²+mgh1 = ½mv2²+mgh2

    ½mv1²= mgh2

    h2 = 3.27m = 3m (is this correct?)

    im really confused with B... coz i got 8m/s... it doesn't make sense to me how they can run at the same speed and achieve the same height??!!

    this is really urgent... so please help me!! >_< thanks!
    Last edited: Mar 22, 2004
  2. jcsd
  3. Mar 23, 2004 #2
    I would say: the KE at the point of him grabbing it = the PE at his peak(all his KE has become PE) so KE=PE so ½mv²=mgh substituting:

    B)change 30 for 40 above and v is unknown

    yes, it is the SAME v for both! Why? because the m cancels out on both sides of the equation: the m in ½mv² and the m in mgh divide out!
    Last edited: Mar 23, 2004
  4. Mar 23, 2004 #3
    thank you!! ^^
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook