Students should learn that when an object floats the water is pushing upwards on the object. For example, when you try to push a surfboard under the water you can feel the water pushing the board up. Objects such as rocks that have sunk, are still experiencing an upwards push; it is just not as strong as the weight force.
Students should be encouraged to view floating as being a result of the balance of the weight force on an object and the upward push of the water on the submerged part of the object. At this level it is appropriate for students to experiment with a variety of objects to see if they float or sink in water.
They should be encouraged to identify common characteristics of objects that float and those that sink. Students can try submerging a ball in a bucket of water in order to feel the upward push of the water on the ball.
A more quantitative approach could involve measuring the weight of different objects to see whether that affects how they float or sink in water. Students could experiment with different sized objects with the same weight to see how this influences their floating. Expected results The clay has a greater mass than an equal volume of water.
Ask students: Which weighs more, the clay or an equal volume of water? The clay weighs more than an equal volume of water. Which is more dense, clay or water? Clay is more dense. Knowing the density of an object can help you predict if it will sink or float in water. If an object is more dense than water, would you expect it to sink or float? Objects that are more dense than water sink. If an object is less dense than water, would you expect it to sink or float?
Objects that are less dense than water float. Explain Compare the density of wax, water, and clay on the molecular level. Project the image Wax. Project the image Water. Project the image Clay. Extend Have students explain, in terms of density, why a very heavy object like a big log floats and why a very light object like a tiny grain of sand sinks.
Ask students: A giant log can float on a lake, while a tiny grain of sand sinks to the bottom. Explain why a heavy object like the log floats while a very light grain of sand sinks. Students should recognize that a log will float because wood is less dense than water.
If you could weigh a large amount of water that has the same volume as the log, the log will weigh less than the water. Therefore, the log floats. A grain of sand will sink because sand is more dense than water.
If you could weigh a small amount of water that has the same volume as the grain of sand, the sand will weigh more than the water. Therefore, the sand sinks. Table 1. Wrong Track: Well that's easy, heavy things sink and light things float. Right Lines: Floating and sinking depend on the density of the material. For equivalent volumes of substances the statement heavy things sink and light things float is close to being true. However, pupils faced with a heavy block of wood, perhaps too heavy for them to lift, and a lightweight paper clip, will make the wrong predictions.
They will predict that floating and sinking depends on the gravity force and not on an intrinsic property of the material, which is the density.
Tangled up in this misunderstanding is the issue of density and also the language we use to compare things. What do larger or bigger mean? To say that a block of metal sinks in water because the mass is greater is only part of the explanation. Its mass is greater than the mass of the same volume of water. Download the student activity sheet and distribute one per student when specified in the activity.
The activity sheet will serve as the Evaluate component of the 5-E lesson plan. Students will record their observations, and answer questions about the activity on the activity sheet.
If students dip the tiny piece of clay in the water beforehand and then put it back on the surface of the water, it should sink. Show the Animation — Density: Clay and Water. Explain that density has to do with how heavy something is compared to its size. As you show the animation, explain that since a piece of clay weighs more than the same amount, or volume, of water, clay is more dense than water. Since clay is more dense than water, a ball of clay sinks in water, no matter how big or small the ball of clay is.
Show the Animation — Density: Wood and Water. If you compared the weight of wood and an equal amount, or volume, of water the sample of wood would weigh less than the sample of water.
This means that wood is less dense than water. Since wood is less dense than water, wood floats in water, no matter how big or small the piece of wood is. The key to floating is being light for your size. So if you can add size to an object without adding much weight, the object will be lighter relative to its size. This means that the density of the overall object will decrease and be more likely to float. Ask students to describe how this principle can be used to explain how a lifejacket can help someone float in water.
The key to sinking is being heavy for your size. If you can add weight to an object without adding much size, the object will be heavier relative to its size. This means that the density of the overall object will increase and be more likely to sink. Ask students to explain how this principle can be used to explain how a weight belt can help a SCUBA diver sink in water when they might otherwise float.
Note: A student might want to know why a boat made out of steel can float when steel is more dense than water.
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