Physics 20 - dynamics

  1. I have been absent from my physics class for the past week due to an illness, and now am having troubles with some of my workbook questions. There are two in particular which my teacher had told me to know how to do as they would be on my test in the next few days.

    the ones he wrote out were...


    a 25 n force will accelerate a 15kg object over a horizontal and low friction surface from rest to 2.4 m/s in 1.4 s find the work done on the object.


    and

    a 25 n force is applied to a 15kg cart where the force of friction is 8.5N the distance travelled is 16.5 m the cart starts from a dead stop. Determine the carts final velocity using its kinetic energy.


    any help would be great.
     
  2. jcsd
  3. HallsofIvy

    HallsofIvy 40,203
    Staff Emeritus
    Science Advisor

    There are two ways to do this.
    The hard way: You know the force and the mass so you can find the acceleration (F= ma). Once you know that, you know how far the object went in 1.4 s and work= force* distance.

    The easy way: You know that the object had initial speed 0 so initial kinetic energy 0. You know the mass and final speed so you can calculate the final kinetic energy- kinetic energy= (1/2)*mass*velocity2. That change in kinetic energy IS the work the force did on the object.

    You know the applied force and you know the friction force so you can subtract to get the "net" force. You know the distance the force was applied and work equals force times distance so you can calculate the work done on the object. That work is converted into kinetic energy and kinetic energy equals (1/2)*mass*velocity2.
     
  4. wow thanks a lot, I'm doing the two questions as we speak and having no problems at all. :smile:
     
  5. just to double check the first question is 43.2 J, and the second 33 m/s?

    please confirm or if if I'm wrong tell me.
     
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