Results 1 to 4 of 4

Math Help - Elevator work, power [Physics]

  1. #1
    Newbie
    Joined
    Sep 2008
    Posts
    10

    Elevator work, power [Physics]

    I threw this one in the physics help forum, but nobody seems to go there so i figured i would try the other topics forum ^^

    A 650-kg elevator starts from rest. It moves upward for 3.00 s with constant acceleration until it reaches its cruising speed of 1.75 m/s.

    (a). What is the average power of the elevator motor during this time interval?


    Couldn't i use the total work thru kinetic energy? W = 1/2 mVf^2 - 1/2 mVi^2 ?
    I tried that the first time for this problem, teacher said that i'm wrong. I'm not sure what to do other than ths.



    (b) How does this power compare with the motor power when the elevator moves at its crusing speed?



    As always, any help is Greatly appreciated....
    Follow Math Help Forum on Facebook and Google+

  2. #2
    -1
    e^(i*pi)'s Avatar
    Joined
    Feb 2009
    From
    West Midlands, England
    Posts
    3,053
    Thanks
    1
    Quote Originally Posted by 92stealth View Post
    I threw this one in the physics help forum, but nobody seems to go there so i figured i would try the other topics forum ^^

    A 650-kg elevator starts from rest. It moves upward for 3.00 s with constant acceleration until it reaches its cruising speed of 1.75 m/s.

    (a). What is the average power of the elevator motor during this time interval?


    Couldn't i use the total work thru kinetic energy? W = 1/2 mVf^2 - 1/2 mVi^2 ?
    I tried that the first time for this problem, teacher said that i'm wrong. I'm not sure what to do other than ths.



    (b) How does this power compare with the motor power when the elevator moves at its crusing speed?



    As always, any help is Greatly appreciated....
    a) Assumption: The motor is 100% efficient.

    You can use the equations of motion to find both distance and acceleration:

    a = a, u= 0, v= 1.75, t= 3

    v = u + at

    a = \frac{v-u}{t} = \frac{1.75}{3} = \frac{7}{12}

    u=0, v= 1.75, t=3, s=s

    s = \frac{1}{2}(u+v)t = \frac{1}{2}(1.75+0)(3) = \frac{21}{8}

    Since a and s are known you can use

    F = ma and W = Fs so W = mas  = 650 \times \frac{7}{12} \times \frac{21}{8} = 995J (3sf)
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Newbie
    Joined
    Sep 2008
    Posts
    10
    Exactly the answer i got, just got there a different way, he said that i was wrong. Wonder if my teacher is the one that looked at it wrong...thanks man
    Follow Math Help Forum on Facebook and Google+

  4. #4
    MHF Contributor
    skeeter's Avatar
    Joined
    Jun 2008
    From
    North Texas
    Posts
    11,621
    Thanks
    426
    Quote Originally Posted by 92stealth View Post
    Exactly the answer i got, just got there a different way, he said that i was wrong. Wonder if my teacher is the one that looked at it wrong...thanks man
    I'm afraid your thinking is wrong ... and your teacher is correct.

    W_{motor} = \frac{1}{2}m(v_f^2 - v_0^2) + mg\Delta y

    W_{gravity} = -mg \Delta y

    W_{net} = \frac{1}{2}m(v_f^2 - v_0^2)

    (a). What is the average power of the elevator motor during this time interval?
    P_{avg} = \frac{W_{motor}}{\Delta t}

    (b) How does this power compare with the motor power when the elevator moves at its cruising speed?
    it's greater ... at cruising speed P = Fv = mgv
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Physics Spring problem. Work is shown!
    Posted in the Calculus Forum
    Replies: 3
    Last Post: March 19th 2011, 02:11 PM
  2. Physics question: Work done by gravitational force
    Posted in the Math Topics Forum
    Replies: 4
    Last Post: November 15th 2009, 01:39 PM
  3. Physics - Power
    Posted in the Math Topics Forum
    Replies: 2
    Last Post: March 5th 2008, 03:38 PM
  4. Physics - Power
    Posted in the Math Topics Forum
    Replies: 2
    Last Post: March 5th 2008, 11:28 AM
  5. Physics work and energy
    Posted in the Math Topics Forum
    Replies: 1
    Last Post: November 6th 2007, 03:11 AM

Search Tags


/mathhelpforum @mathhelpforum