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: Heat and energy

  1. Feb 21, 2014 #1
    1. The problem statement, all variables and given/known data

    Rubbing your hands together warms them by converting work into thermal energy. If a woman rubs her hands back and forth for a total of 10 rubs a distance of 7.50 cm each and with a frictional force averaging 45.0 N, what is the temperature increase? The mass of tissue warmed is only 0.100 kg, mostly in the palms and fingers.

    2. Relevant equations

    work = force * distance

    3. The attempt at a solution

    i can find the work, but after that I don't know how to connect that to temperature change.
  2. jcsd
  3. Feb 21, 2014 #2
    Some of the work is converted to heat, I would go with Q = cmΔT , but I have no idea what c is for human skin, tissue, cells or whatever. If you assume the process is 100% efficient, all the work done goes toward heating the skin, then all you would need is the c , the amount of energy it takes to heat up 1 kg of mass by 1 K
    It is said lots of it in our body is water, the c for water is 4.2 kJ/ kgK, the surface of the skin is sweat so water again, but I'm not a beauty salon worker :/, can't help you any further at the moment.
    Last edited: Feb 21, 2014
  4. Feb 21, 2014 #3


    User Avatar
    Gold Member

    Hi Naruto,
    Specific heat capacity of human body on average is 3470 J/(kg⋅°C).So I think you should use that.
  5. Feb 21, 2014 #4
    so does all the work convert to heat?
  6. Feb 21, 2014 #5
    Never does. There is no further data on the assignment, therefore I can assume the process is 100% efficient.
  7. Feb 21, 2014 #6
    Thanks guys I got it.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted