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Homework Help: PHYSICS work/power/energy question

  1. Oct 7, 2005 #1
    8. For each statement, write true or false.
    A)only the net force acting on an object can do work.
    i think its TRUE.
    B)no work is done on a particle that remains at rest.
    i think its TRUE.
    C)a force that is always perpendicular to the velocity of a particle never does work on the particle.
    i think its TRUE.

    ^ please correct me if im wrong.. ^ thx.

    6. A particle of mass m moves along the x axis. Its position varies with time according to x=2t^3 + 4t^2 where x is in meters and t is in seconds.
    B)find the power delivered to the particle at any time t.
    ????
    C)the work done by the force from t=0 to t=11
    im guessing i have to integrate the equation 2t^3 + 4t^2 with 11 on top right of the integration sign and 0 on the bottom of the integration sign.
    any comment will be hhelpful. thx in advance!

    10. If a ski lift raises 100 passengers averaging 660N in weight to a height of 150m in 60s, at constant speed, what average power is required of the force making the lift?
    I got P = W/t = mgh/t = 660N*150m/60s = 1650W = 1.6kW
    did i do this right?
     
    Last edited by a moderator: Oct 7, 2005
  2. jcsd
  3. Oct 7, 2005 #2

    Andrew Mason

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    Think about two equal and opposite forces compressing a volume of gas. The net force is 0. But there is work done ([itex]W=P\Delta V[/itex]).
    See example above.
    True. Since [itex]W = \int \vec{F}\cdot \vec{s} = \int F\cdot \vec{v} dt = \int Fvdt cos\theta[/itex] if [itex]\theta[/itex] is always 90 degrees, then W = 0.

    Use [itex]P = dW/dt = Fds/dt = Fv = mav = m\ddot x \dot x[/itex]
    See suggestion in B)

    The work done in raising 100 passengers is W=mgh where mg = 100*660N and h = 150 m. So the correct answer is 165000 W. or 165 kW.

    AM
     
  4. Oct 10, 2005 #3
    um.. mg = 660 m^2/s^2.. then multiply this by h=150m. I dont understand how you could put 100 as its mass...
     
  5. Oct 10, 2005 #4

    Andrew Mason

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    660 N is the weight of one person (average). There are 100 people. So the total weight is 100*660.

    AM
     
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