1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Simple energy/work

  1. Jan 27, 2010 #1
    1. The problem statement, all variables and given/known data
    A 890 N crate rests on the floor.
    How much work is required to move it at constant speed 5.6 m along the floor against a friction force of 180 N?
    How much work is required to move it at constant speed 5.6 m vertically?

    2. Relevant equations
    KE = (1/2)mv^2


    3. The attempt at a solution
    (1/2)(890/9.8) = x?
     
  2. jcsd
  3. Jan 27, 2010 #2
    You don't know the velocity so kinetic energy won't help you. Any other equations you can think of?
     
  4. Jan 27, 2010 #3
    Work force done = FaDcos(theta)
    Where:
    Fa = applied force
    D = displacement
    Cos(theta= im not sure would it be pie/2
     
  5. Jan 27, 2010 #4
    cos(pi/2) = 0, here your work is being done in the same direction as the motion so.

    That equation gets you half way there, need one more equation.
     
  6. Jan 27, 2010 #5
    I think cos(pi/2) = 1.?
    Mhm is the other equation:
    Work done by friction = FfD
    Where Ff= friction force
    D = displacement?
     
  7. Jan 27, 2010 #6
    Actually I wasn't quite right all you need is the equation you just wrote sorry bout that. So just use the equation you just wrote, (W done by friction) = (F of friction)*d
    Then just plug and chug.
     
  8. Jan 27, 2010 #7
    Thanks I was able to answer part 1.
    How about:
    How much work is required to move it at constant speed 5.6 m vertically?

    So here there is a gravitational component?
     
  9. Jan 27, 2010 #8
    W=f*d, you know the distance. Can you calculate the force due to gravity?
     
  10. Jan 27, 2010 #9
    The force due to gravity is just mg, (860)
    W = 890*5.6?
     
  11. Jan 27, 2010 #10
    there ya go easy right? In general if you have an equation with n variables you need n-1 equations to solve for the one variable you're looking for.
     
  12. Jan 27, 2010 #11
    Thanks, and thanks for the tip!
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook