1. Not finding help here? Sign up for a free 30min 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!

Time it takes to lift a sofa with power

  1. Oct 4, 2011 #1
    1. The problem statement, all variables and given/known data

    Your sofa won't fit through the door of your new sixth-floor apartment, so you use a 1.18 kW motor to lift the 84.7 kg sofa 18.8 m from the street.

    How much time does the lift take?

    2. Relevant equations

    KE_i+PE_i=KE_f+PE_f
    (If forces are conserved? I'm not sure if so.)

    3. The attempt at a solution

    KE_i=PE_f
    1/2mv^2=mgh
    1/2v^2=gh
    This leads to a dead end I think, velocity ends up being very large..
     
  2. jcsd
  3. Oct 4, 2011 #2

    SammyS

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    Presumably, the sofa is lifted at a constant speed, so its kinetic energy doesn't change.
     
  4. Oct 4, 2011 #3
    So conservation of energy would apply, so the Kinetic energy will equal the potential energy of the sofa at the top?

    With the conservation equation how would I get to the units of time?
     
  5. Oct 4, 2011 #4

    SammyS

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    I repeat, the Kinetic Energy doesn't change. It remains the same.

    What is power?
     
  6. Oct 4, 2011 #5
    Power is work over time.
     
  7. Oct 4, 2011 #6

    SammyS

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    If the work doesn't go into increasing the K.E., then What form of energy is being increased as the piano is lifted?
     
  8. Oct 4, 2011 #7
    The potential energy increases..
     
  9. Oct 4, 2011 #8

    SammyS

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    Right.

    So how much energy must the sofa gain to make it to the 6th floor?
     
  10. Oct 4, 2011 #9
    Found it, I need to gain a better grasp for what my units are. Thank you for getting me started on it. I found the PE=mgh and that is the work done.

    t=w/p gets me my answer.
     
  11. Oct 4, 2011 #10

    SammyS

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    Yup. That's it.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Time it takes to lift a sofa with power
Loading...