1

I have two water droplets, about 2mm across, one with an initial velocity of $4ms^{-1}$ towards the ground, which is 5 metres away. The other has an initial velocity of $0ms^{-1}$. Both have an average temperature of 50°C.

I'm not sure how I can calculate the energy lost to air resistance, and how much of this energy is lost as a reduction in acceleration, and how much is a reduction in temperature.

I've calculated the speeds when the drop hits the floor assuming there is no air resistance:

Which droplet is colder? (This is not a homework question).

John Rennie
  • 355,118
Tim
  • 878
  • 1
  • 6
  • 15
  • 1
    It may not be a homework question but it is formulated like one and you have shown no effort to tackle the problem. Voting to close. – JamalS Apr 12 '17 at 19:14
  • @JamalS I've got some calculations- would a photograph of those be suitable? – Tim Apr 12 '17 at 19:14
  • No, you need to write it up, preferably using LateX. – JamalS Apr 12 '17 at 19:15
  • @JamalS I'm on mobile - latex is shockingly difficult to type. Why is a photograph unsuitable? – Tim Apr 12 '17 at 19:16
  • @Tim: E.g. because text in pics are not searchable. – Qmechanic Apr 12 '17 at 20:32
  • Possible duplicate of Why does water get significantly colder while falling through the air? – sammy gerbil Apr 12 '17 at 20:49
  • @sammygerbil no, I'm asking which is cooler. Not why they're cooler! They're cooler because of air resistance... – Tim Apr 12 '17 at 20:50
  • I think it's a good qustion. It is well-defined and non-trivial. It's not a duplicate of the suggested since it is quantitative and specific rather than "generally-speaking". If you'll find the time to format it properly and show an attempt to find the heat loss, you should attract better responses. I'd actually like to know the answer myself. – Yoni Apr 12 '17 at 21:01
  • You have shown some effort to calculate the speed at which the drops reach the ground. But how do you think this affects the answer to your question? The question I cited asks What is a good mathematical and thermodynamic model for explaining the temperature change of falling water? Without such a model it is difficult to make a calculated prediction. Even a qualitative answer is difficult : it is not obvious whether the faster droplet gains more heat from friction than it loses from evaporation. – sammy gerbil Apr 12 '17 at 21:20
  • @sammygerbil well that's part of my question - does it gain or lose - either could. does the slower one lose more because it's in the air longer or less because it's slower. – Tim Apr 12 '17 at 21:28

0 Answers0