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!

Efficiency question

  1. Feb 1, 2017 #1
    1. The problem statement, all variables and given/known data
    A motor lifts a block of mass 0.050kg at a constant velocity of 0.40m s-1. The current in the motor ia 85mA and the potential difference across it is 3.0V. Calculate the efficiency of the motor

    2. Relevant equations
    Ek = 1/2 x mv2
    Electrical power = VIt

    3. The attempt at a solution
    Ig to the electrical power as 0.255W, and kinetic energy of the block as 0.004J. But how do I get the power of the block, or the electrical energy of the motor?
     
  2. jcsd
  3. Feb 1, 2017 #2

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Why do you think the kinetic energy of the block is important?
     
  4. Feb 1, 2017 #3
    Isn't efficiency useful (energy/power)/total (energy/power) x100
    And kinetic energy is the useful energy.
     
  5. Feb 1, 2017 #4

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Is the kinetic energy changing over time?

    What if the block were moved horizontally at constant speed?
     
  6. Feb 1, 2017 #5
    It is lifted at a constant velocity.
     
  7. Feb 1, 2017 #6

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    So, what's the change in Kinetic Energy?
     
  8. Feb 1, 2017 #7
    0
     
  9. Feb 1, 2017 #8

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    So, what energy is changing?
     
  10. Feb 1, 2017 #9
    Electrical to kinetic. Since I'm only given the block's mass, velocity, the current and voltage of the motor, I can't do power of the block, of the electrical energy of the motor.
     
  11. Feb 1, 2017 #10

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    No. Kinetic energy is not changing. We have already established that.

    Why is it hard to lift something heavy?
     
  12. Feb 1, 2017 #11
    How is it hard to lift 0.050kg (50g)? All I want to know is how to get the efficiency of a motor using the mass and velocity of a block, and the current and voltage of the motor.
     
  13. Feb 1, 2017 #12

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    I'll give you a big hint. At some stage, however, you are going to have to learn to think for yourself.

    Hint: you're completely forgetting about gravity.
     
  14. Feb 1, 2017 #13
    But what do I do with it, there is no given height.
     
  15. Feb 1, 2017 #14

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    I'm sorry, but you need to look at my last post. You need to think about the problem.
     
  16. Feb 1, 2017 #15
    Do I do 0.4/9.81 to get the time in seconds, then times it by the current and voltage to get electrical energy. Then put that under kinetic energy, and times it by 100? It gives 38.5%
     
  17. Feb 1, 2017 #16
    Time to lift the mass is required
    Kinetic energy /time gives you the power, then you compare the motor power with the calculated power to find efficiency of this system
     
  18. Feb 1, 2017 #17

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    I don't really understand why you are still trying to work with the kinetic energy. It's the gravitational potential energy of the block that is changing.
     
  19. Feb 1, 2017 #18

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    This is nonsense!
     
  20. Feb 1, 2017 #19
    Oh...
     
  21. Feb 1, 2017 #20
    0.4/9.81 = 40/981s
    0.4x40/981=16/981m
    9.81x0.05x16/981=1/125J
    (100/125)/(0.085x3x(40/981)) = 76.94% = 76.9%
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Efficiency question
  1. Efficiency Question (Replies: 7)

  2. Efficiency question. (Replies: 1)

  3. Efficiency Question (Replies: 1)

  4. Efficiency question (Replies: 1)

Loading...