|MadSci Network: Physics|
G'Day, Tom (and Scott)! You've asked a really good question, and I've got to admit that it stumped me for a while... I'm really impressed by the thought and analysis you've put into the question already. You've given the color some thought, and you recorded the temperature using a thermometer rather than trying to make a comparison using your hand. From a thermodynamics standpoint, a metal object will gain and lose heat faster than a plastic object. Heat is transfered out of an object by three mechanisms: Conduction (heat flow through direct contact), Convection (heat flow through secondary contact with a fluid such as air or water), and Radiation. (heat being emitted as waves of energy.) Metals are better conductors of heat (and electricity) because of the free electrons they have within them. So plastic (an insulator) will hold heat in better and longer than a metal. Therefore, if you heat a metal object and a plastic object to the same temperature, and allowed both of them to cool off, the metal object should warm up faster, but also will cool down faster. Did you take your temperature data in the afternoon? If so, and if the sun heated up both the plastic and the metal slides, the plastic will "hold" in it's heat longer, even though it probably took a longer time to get warm. So when the temperature drops, the metal will cool off faster, and the metal slide will be cooler than the plastic slide. You also mentioned the size and shape of the playground equipment, and that's probably a major factor as well. It's a lot easier and cheaper to make metal objects using hollow pieces or thin sheets. With less mass and lots of surface area, the object will lose heat a lot more rapidly than a thicker, more solid object of the same material. If the plastic slide was thicker and more solid (I would guess it would have to be to be strong enough) than the metal slide, then that's another reason why the plastic one will stay warm longer. Stuff to try: 1) Try take a few more temperature measurements as the day progresses. Try some in the morning, some at the hottest part of the day, and some at night. If you make a graph and plot the temperature versus the time, you'll probably see a cyclic pattern for both the metal and the plastic slides. You'll also be able to pinpoint at which times of the day the plastic slide will be warmer. (This data will be especially handy on days when you forget your coat...) 2) Take a plastic fork, and a metal fork, and leave them on a table (away from sunlight) for a little while. If you measure the temperature, they should be about the same. (The metal fork will feel colder to your hand because it conducts heat away from your hand faster then the plastic one.) Then drop one end of one of the forks into a cup of water with ice in it. Take temperature measurements from the other end every ten seconds or so. (Use a stopwatch or a clock with a second hand if you need to) Then try it with the other fork. You'll be able to see that the metal fork cools down faster than the plastic one. Next, you can put one end of the fork in a cup of warm water, (ask a parent if you need help heating up the water) and try the same thing. You'll see that the metal fork heats up faster as well... I hope this helps! And I hope you keep thinking up good questions!
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