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Homework Help: Kinematics and dynamics

  1. Jul 16, 2013 #1
    1. The problem statement, all variables and given/known data

    the question is:
    A 62 kg person in an elevator is moving up at a constant
    speed of 4.0 m/s for 5.0 s. T / I C

    (b) Calculate the work done by the normal force on the person.
    (c) Calculate the work done by the force of gravity on the
    (d) How would your answers change if the elevator were
    moving down at 4.0 m/s for 5.0 s?

    2. Relevant equations
    W= F x Δd

    and i think

    Δd= (vf + vi/2)Δt
    3. The attempt at a solution

    for b) i did Δd= (vf + vi/2)Δt and got displacement which is 10 m and then i did fn=fg since fnet is 0 which means that they have the same force. and for that i got 607.6 N

    p.s the answer in the back is 12 kJ which i dont get?
  2. jcsd
  3. Jul 16, 2013 #2
    At constant velocity v, what is displacement d over time t?
  4. Jul 16, 2013 #3


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    Homework Helper

    The lift was moving at a constant speed, so calculating displacement is much simpler than the way you did it - and your method gives the wrong answer because you assumed it started out at 0 m/s.
    This 5 second interval is presumably a middle interval of the whole journey.
    The lift gains speed - then travels at constant speed - then slows down and stops. The 5 sec interval is entirely in the middle section.
  5. Jul 16, 2013 #4


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    Science Advisor
    Homework Helper
    Gold Member

    I believe you meant Δd= [(vf + vi)/2)]Δt. If the speed is constant, vf and vi are the same. Try again.
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