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Homework Help: Work and Energy

  1. Oct 20, 2005 #1
    Please let me know if I am on the right track for these problems.
    1) A weight lifter picks up a barbell and
    1. lifts it chest high
    2. holds it for 5 minutes
    3. puts it down.
    Rank the amounts of work W the weight lifter performs during this three operations. Label the quantities as W1, W2, and W3. Justify you ranking order. ---> I think that the most amount of work is when it is lifted chest high, since there is a force and displacement involved. After that, would me when he puts it down, becuase gravity helps, and there is a displacement. When he holds it up for five min there is no work becuase there is no displacement.
    2)Estimate the amount of work the engine performed on a 1200 kg car as it accelerated at 1.2 m/s2 over a 150 meter distance. ----> Well since the equation for work is force times distance, it would be (1.2 m/s2)(1200kg)(150m) So the total work would be 216,000 J
  2. jcsd
  3. Oct 20, 2005 #2
    I think you are right for both questions, you've used the equations:

    [tex]Wd=F \times d[/tex] and
    [tex]F=m\times a[/tex]


    Wd is work done in Nm (Newton meters)
    F is the force in N (Newtons)
    d is the distance in m (meters)
    m is the mass in kg (kilogrammes)

    a is the acceleration in [tex]\frac{m}{s^2}[/tex] (meters per second squared)

    I think you have interpreted and used these equations correctly.
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