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Assessment
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Have students recall that in the first investigation, they compared soil to sand. Sand, they observed, was made of particles that appeared to be very similar. Use this experience to measure their understanding of separating a soil mixture by asking: “Is sand a mixture?” (No, all of the particles appear to be very much the same.) “What do you think would happen if we tried your separation methods with a sample of sand?” Ask them for the reasoning behind their predictions. (If students understand the concepts behind separation of mixtures, they will predict that sand, with its similar particle sizes, will fail to separate by the soil separating methods.)
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Revisiting Investigation Question 2: How can you separate a soil mixture?
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Complete this investigation by asking your students to reflect on the investigation question and how their answers may have changed as a result of what they have learned. Ask them why they think it’s important to know what is in a soil mixture. [Knowing what’s in soil is important for planting crops, preventing erosion, having water move through soil and supporting structures, among many other things.]
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It’s important to recognize a common misconception that most of us share about falling objects, whether through air or water: It seems perfectly natural to us that in air, an object like a bowling ball with its large mass and size falls to the Earth faster than a feather. Both objects are falling toward the Earth’s center because of the pull of gravity. But the explanation for one falling faster than the other is complicated. It’s hard to believe it, but if both were released from a given height above the surface of the moon, they would reach the surface in exactly the same amount of time! The reason for our very different result on the Earth’s surface is explained by the atmosphere—air—through which they fall. In air, the object that exposes the most surface area contacts the most air. And that contact produces a braking effect called friction. So why don’t a bowling ball and a balloon inflated to the same size reach the Earth at the same time? The mass of the bowling ball gives it momentum to counteract the force of friction to a much greater extent than the mass of the much lighter balloon. In these investigations, the friction was between the various soil particles and the medium through which they were poured. More dense elements reached the bottom of the container faster than less dense elements. And the effect was more marked in methods that used water because the force of friction is greater between objects and water than between objects and air. |
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