The Playground Slide and the Sun-Baked Metal
A class tests surface temperatures on a playground at 1:00 PM on a sunny 28°C day. They place identical thermometers on four surfaces for five minutes: a plastic slide (35°C), a metal handrail in direct sun (47°C), a wooden bench in shade (24°C), and a rubber mat in direct sun (38°C). A student says, "The metal is the hottest because metal is naturally a hot material." Another says, "The handrail just happened to be in the sun." The teacher asks the class to design a fair comparison to figure out what actually causes the temperature differences. Materials available: two identical metal bars, two identical plastic bars, a sunny table, a shaded table, and thermometers.
1. Describe a fair test the class could run to decide whether the material or the sunlight is the main cause of the handrail's high temperature. State what to change, what to keep the same, and what to measure.
Read the worked answer
Change (independent variable): the material of the bar (metal vs. plastic), while placing both in the same sunlight.
Keep the same (controlled variables): size and shape of the bars, starting temperature, time in the sun, and the surface they rest on.
Measure (dependent variable): temperature of each bar after the same time interval.
Setup: Put one metal bar and one plastic bar on the sunny table, and the other metal and plastic bars on the shaded table. Record starting temperatures, wait the same number of minutes, and record again.
Reasoning: By comparing metal-in-sun to plastic-in-sun, any difference must come from the material. By comparing metal-in-sun to metal-in-shade, any difference must come from the sunlight. This separates the two possible causes the students argued about.
2. The class finds that, in the shade, metal and plastic bars end up at nearly the same temperature, but in the sun the metal bar heats up faster. Does this support the claim that "metal is naturally a hot material"? Explain using the evidence.
Read the worked answer
No. The shade result is the key evidence: with no sunlight, metal and plastic reached nearly the same temperature, so metal is not 'naturally hot.'
The sun result shows metal heats faster when energy from the sun is added, but that is about how quickly a material takes in and spreads heat, not about it being hot on its own.
A complete answer notes that temperature depends on both the material and the energy source (sunlight). The original playground data fits this: the shaded wooden bench was coolest (24°C) even though it was outdoors, and the metal handrail was hottest (47°C) because it was both metal and in direct sun.
3. Predict what would happen to the metal handrail's temperature if a cloud blocked the sun for ten minutes. Justify your prediction with the pattern from the investigation.
Read the worked answer
The handrail's temperature should drop toward the shaded metal bar's temperature, because the energy input from sunlight would be removed.
Justification: In the investigation, metal in shade stayed near the plastic-in-shade temperature and well below metal in sun. That shows the high reading depended on continuous sunlight, not on the metal alone.
A strong answer predicts a gradual drop rather than an instant one, since the metal holds some of the energy it already absorbed, and it may not fall all the way to the shaded value if the cloud is thin or brief.