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Homework Help: Power and maximum power problem

  1. Nov 19, 2004 #1
    A loaded ore car has a mass of 950kg and rolls on rails with negligible friction. It starts from rest and is pulled up a mine shaft by a cable connected to a winch. The shaft is inclined at 30 degrees above the horizontal. the car accelerates uniformly to a speed of 2.2m/s in 12 sec and continues at constant speed. (a) what power must the winch motor proide when the car is moving at constant speed ? (b)what maximum power must the winch motor provide?

    I know (A) is....
    P= F*V
    P=mgsin30 * V
    Know how do you do (b) i think its similar to (a) i dont know what to do
  2. jcsd
  3. Nov 19, 2004 #2
    I messed up on the last line... i meant i dont know how to do (b) but i think its like (a)
    can any one shed some light
  4. Nov 19, 2004 #3
    ohhhh man! does anyone have any idea???? haha

    if its asking for the maximum power what would change. the mass is the same so is g the velocity im not sure about but what would it change to?? the sin im certain would stay the same aswell. I dont really understand the differene between part a and b
    Last edited: Nov 19, 2004
  5. Nov 19, 2004 #4
    F = ma

    Thus, when the car is accelerating, the force acting on the car not only must overcome the force of gravity, but it also must accelerate the car.
  6. Nov 19, 2004 #5
    what??? well if you find the F that way then....
    F=ma (1st i need acceleration.....(2.20m/s / 12sec)= .183m/s^2
    F=950kg*.183m/s^2 F=174N
    So then...
    P=174sin30*2.2 P=191W

    but the answer is P=10600W
  7. Nov 19, 2004 #6
    Watch what you do....
    it is not F=ma
    it is Sum(F) = ma

    YOu have the cable pulling in one direction and the weight coming down in another...

    thuse your eq is:

    T - mgsin(30) = ma

    Where T is the tension and thus the force in the cable

    Make sure you look at the free body diagram :)
  8. Nov 19, 2004 #7
    yeah...sorry 'bout that..I left out the sumation part... but basically follow Spectre5's advice...free body diagrams are essential to solving most force related Physics problems
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