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Test Tomorow on work, and more

  1. Jan 15, 2007 #1
    I have a test tomorrow and i lost all my papers on how to do this stuff. And i have this paper that has examples of all the problems i need to know how to do.

    1. The problem statement, all variables and given/known data
    1. A humming bird has a mass of 1.70 g. if the hummingbird ascends straight up with a net acceleration of 1.20 m/s^2, how much work does it do over a distance of 8 m.

    2. The longest shish kabob ever made was 881 m long. Suppose the meat and vegetables need to be delivered in a cart from one end of this shish kebab to the other end. A cook pushes the cart by applying a force of 40.0 N at an angle that is 45ยบ above the horizontal. If the force of friction acting on the cart is 28 N what is the net work done on the cart and its contents during the delivery.

    2. Relevant equations

    3. The attempt at a solution
    1. (.0017kg) / (1.2 m/s^2) = .0014 N
    .0014 x 8m = .0113 J

    What am i doing wrong?

    2. I have know idea how to do this one.
  2. jcsd
  3. Jan 15, 2007 #2


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    Use the net acceleration to calculate the net force.

    Thenm, taking into account the weight of the bird, you can calculate the upwards force it's generating.

    use this in your W = Fd equation.
  4. Jan 15, 2007 #3


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    Staff Emeritus
    Science Advisor

    1. The correct equation is F=ma.

    2. Draw a free body diagram. This will enable you to visualise the situation, then resolve forces into horizontal and vertical components.
  5. Jan 15, 2007 #4
    for the humming bird:
    as the bird goes vertically upwards, therefore..

    when m=mass of birdy,
    g= acceleration due to gravity(9.8 m/s^2), and
    a=acceleration of bird(1.20 m/s^2)

    so net force applied by bird will be: F=m(g+a)
    =0.0017(9.8+1.2)=0.0187 N
    Thus workdone= F*s--> distance

    << explicit solution removed by berkeman and solution attachment deleted >>

    {refer the pic.}
    Draw a free body diagram and it will help you!!
    Last edited by a moderator: Jan 15, 2007
  6. Jan 15, 2007 #5


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    Science Advisor

    First question: calculate mgh, the change in potential energy. Using the given acceleration find the final velocity and so (1/2)mv2, the change in kinetic energy. The change in total energy is the work done.

    Second question: Work= force times distance. Since the distance is measured horizonatally, use the horizontal component of force.
  7. Jan 15, 2007 #6


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    Staff: Mentor

    Welcome to the PF, Rocket_guy. But you need to keep in mind that the rules here are that we do not post complete solutions to homework problems. Our task is to help the original poster (OP) figure out the problem on their own. We provide hints, pointers, reminders, and other guidance, but not explicit answers. We certainly appreciate your help in the homework forums, but please strive to be more tutorial in your help.
  8. Jan 15, 2007 #7
    Oh .. sry, didnt knew and I was just trying to help becoz the guy has a test tomorrow!

    Should the solution remain there or the post be edited??

  9. Jan 15, 2007 #8


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    Staff: Mentor

    No worries. I think I took out enough of the final solutions and left some of your hints. We'll see what the OP comes back with.
  10. Jan 16, 2007 #9
    What I just noticed is that u deleted the solution to second question without even leaving the hint! Was my answer incorrect??
  11. Jan 16, 2007 #10


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    Staff: Mentor

    I don't remember. Go ahead and repost the hint if you like. I tried to be surgical, but I may have been in too much of a hurry.
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