Physics Help Forum Some fun problems to solve

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 Jan 25th 2017, 10:39 AM #1 Junior Member   Join Date: Jan 2017 Posts: 2 Some fun problems to solve Hi, the teacher gave me this problem to solve. So i hope you can help me with it. Here is the problem: John poured 150ml ob boiling water in 2 cups made out of polystyrene (no heat goes out). In 1 cup he add 5 mm thick layer of hot oil. He measured starting temperatures and then he measured temperatures again after 10 min. He measured the mass of water and water+oil before and after 10 min. Here are the resaults: -cup 1 (only water) starting temp.: 96°C finnish temp.: 64°C starting mass: 149.4g finnish mass: 142.42g -cup 2(with oil on top) starting temp.: 96°C finnish temp.:91°C starting mass: 164.72g finnish mass: 164.70g The surface of the both cups is 0.003 m2 Questions: 1.Why did the water covered with oil cooled down slower that the one with no oil? Show with calculations. 2.Why did John use oil with temperature around 100°C and not oil with room temperature? How would the resaults change if he used room temeprature oil? 3. Which other physical values, which are relevant, can you calculate from this informations? Thank you for your help Last edited by XFACTORMAN; Jan 25th 2017 at 11:11 AM.
 Jan 25th 2017, 11:49 AM #2 Senior Member   Join Date: Aug 2010 Posts: 434 I would think 1. at least would be obvious. If the Styrofoam cup insulates the water, what causes it to cool? For 2. why John does one thing rather than another depends on John' purpose in doing this experiment. For 2., again, what causes the water to cool?
 Jan 25th 2017, 11:55 AM #3 Junior Member   Join Date: Jan 2017 Posts: 2 cups are open on the top so air cools the liquid down. Air temperature is 20°C. i forgot to include that.. Last edited by XFACTORMAN; Jan 25th 2017 at 12:19 PM.
 Jan 25th 2017, 02:06 PM #4 Senior Member     Join Date: Jun 2016 Location: England Posts: 1,006 I note that the cup with only water ends up 7 grams lighter, I would assume that that would be due to evaporation...

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