MadSci Network: Physics

Re: why is some plastic playground equipment hotter than metal?

Date: Mon May 21 18:23:40 2001
Posted By: Jeff Yap, Materials Engineer
Area of science: Physics
ID: 990387472.Ph

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 
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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