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Homework Help: PE and KE - please check my answers

  1. Sep 25, 2004 #1
    1. A 2150-kg elevator carries a 90-kg man from the ground up to the top floor of a building in 45 seconds. In doing so, the elevator develops 73000 W of power. How high was the man carried?

    (73000 W) (45 s) = 3285 kJ
    (9.8 m/s ^2) (2240 kg) = 21952 N

    3285 kJ / 21952 N = 149.64 m



    2. A 600-kg car traveling at 163 km/hr collides head-on with a 1000-kg parked car. If the cars stick together after impact, how fast are they then traveling? What is their kinetic energy?

    (1000 kg) (163 km/h) = (600 kg + 1000 kg) (101.875 km/h)
    ½ (1600 kg) (101.875) = 8302812 J





    3. A 500-g ball rest on top of a tower, 15 meters tall. Calculate its potential and kinetic energies a) when the ball rests on top of the tower, b) after the ball has fallen 10 meters toward the ground, c) 1 meter above the ground.

    a. KE = 0
    PE = (.5 kg) (9.8 m/s^2) (15 m)
    PE = 73.5 J

    b. KE = ½ (.5 kg) (196 m/s)
    KE = 49 J
    PE = (.5 kg) (9.8 m/s^2) (5 m)
    PE = 24.5 J

    c. KE = ½ (.5 kg) (274.4 m/s)
    KE = 68.6 J
    PE = (.5 kg) (9.8 m/s^2) (1 m)
    PE = 4.9 J
     
  2. jcsd
  3. Sep 25, 2004 #2

    Integral

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    The numbers appear to be ok.

    You need to be more careful with your units

    KE = ½ (.5 kg) (196 m/s)
    Should read

    KE = ½ (.5 kg) (196 (m/s)2)
     
  4. Sep 25, 2004 #3
    seems ok to me...

    marlon
     
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