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: Kinetic friction and lost mechanical energy

  1. Feb 12, 2015 #1
    An Object slides with a consistent velocity down an incline that has a 30 degree angle.
    I'm trying to find the kinetic friction and the ratio that the mechanical energy is lost on the way down.

    I think I have already calculated µ with F(friction)= µmgcos(θ)=mgsin(θ) out comes Tan(θ) = µ
    I think I did that correctly..
    I'm struggling with the other part about the rate that the mechanical energy is lost. I would guess that it is not possible to get the answer in a number.
    I tried to us PE = mgh -friction = k2.
    mgh - µmghcos(θ) = ½mv^2þ
    not sure what my answer might look like in the end.
  2. jcsd
  3. Feb 12, 2015 #2
    I agree with you that u= tan(theta). As for the ratio- the ratio of mechanical energy lost to what? Thermal energy maybe?
  4. Feb 12, 2015 #3
    It only says find the mechanical energy that is lost because of friction on the way down the incline.
    I think U =mgs is the potential energy. energy on the end of the incline is K = ½mv^2
    so I think the diffrent between the two is the friction force.
  5. Feb 12, 2015 #4
    Ok so there is no ratio. I think your getting force and energy confused also. Be aware that force has units of Newtons= kg*m/s^2 and energy has units of Joules= N*m= kg*m^2/s^2. The mechanical energy lost is equal to the thermal energy generated. So if T is thermal energy then using conservation of energy set ΔKE+ΔPE+T= 0 and solve for T. btw this should probably not be posted in advanced physics
  6. Feb 12, 2015 #5
    Okey thanks I will try that. New here so I didn't know where to post
  7. Feb 13, 2015 #6


    User Avatar
    Science Advisor
    Homework Helper
    2017 Award

    Well, hello hunbogi, and welcome to PF :)

    I suppose the consistent velocity is a constant velocity and the ratio is the rate, i.e. the mechanical energy loss per unit time.
    There are two ways to find that rate:
    One is friction force times displacement per unit time
    Two is from potential energy loss per unit time
    Answers should be the same
  8. Feb 13, 2015 #7
    The rate of energy dissipation will depend on the velocity. Is the velocity given?
  9. Feb 15, 2015 #8
    no it only gives the angle 30 degree and says the velocity is consistent
  10. Feb 15, 2015 #9
    Consistent velocity does not make much sense. Is this a translation from another language?
    If it's constant velocity, it can have any value, including zero. In which case there is no energy dissipation.
    Something is missing and it may be due to translation.
  11. Feb 15, 2015 #10
    Sorry it is constant velocity. mixed up in translation
  12. Feb 15, 2015 #11
    Can you post the entire problem, as it is? Not just a summary.
  13. Feb 15, 2015 #12
    An object slides with a constant velocity down an 30 degree incline
    a) Find the friction force and find the ratio of the mechanical energy that is lost on the way down
  14. Feb 15, 2015 #13
    Oh, I think you want to say "the fraction of mechanical energy". This could make sense.
    It may ask "What fraction of the mechanical energy is lost due to friction". Like, one half or 1/3 or 0.2.

    Still does not seem completely alright. It is not clear what mechanical energy they are talking about.
    The kinetic energy does not change. So the decrease in potential energy will be equal to the work of the friction force.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted