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Chapter Twelve: Properties of Matter 12.1 Properties of Solids 12.2 Properties of Fluids 12.3 Buoyancy

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12.3 Buoyancy is a force Buoyancy is a measure of the upward force a fluid exerts on an object that is submerged. The water in the pool exerts an upward force that acts in a direction opposite to the boy’s weight.

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12.3 Volume and buoyancy The strength of the buoyant force on an object in water depends on the volume of the object that is underwater. As you keep pushing downward on the ball, the buoyant force gets stronger and stronger. Which ball has more volume underwater?

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12.3 Weight and buoyancy Weight is a force, like any other pushing or pulling force, and is caused by Earth’s gravity. It is easy to confuse mass and weight, but they are not the same. Weight is the downward force of gravity acting on mass. What is the rock’s weight? What is the rock’s mass?

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12.3 Weight and buoyancy In the third century BC, a Greek mathematician named Archimedes realized that buoyant force is equal to the weight of fluid displaced by an object. A simple experiment can be done to measure the buoyant force on a rock with a spring scale when it is immersed in water.

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12.3 Weight and buoyancy In air the buoyant force on the rock is 29.4 N. When the rock was submerged, the scale read 19.6 N. The difference is a force of 9.8 N, exactly the amount of force the displaced water exerts.

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12.3 Weight and buoyancy These blocks are the same total volume. Which block has more buoyant force acting on it? Which block weighs more in air?

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12.3 Weight and buoyancy Buoyancy explains why some objects sink and others float. Whether an object sinks or floats depends on how the buoyant force compares with the weight.

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12.3 Density and buoyancy If you know an object’s density you can quickly predict whether it will sink or float. Which ball will sink in water? Which ball will float in water?

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12.3 Density and buoyancy Average density helps determine whether objects sink or float. An object with an average density GREATER than the density of water will sink. An object with an average density LESS than the density of water will float.

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12.3 Density and buoyancy What can you say about the average density of these blocks?

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12.3 Density and buoyancy When they are completely underwater, both balls have the same buoyant force because they displace the same volume of water.

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12.3 Boats and average density Use your understanding of average density to explain how steel boats can be made to float.

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12.3 Boats and average density If you have seen a loaded cargo ship, you might have noticed that it sat lower in the water than an unloaded ship nearby. This means a full ship must displace more water (sink deeper) to make the buoyant force large enough to balance the ship’s weight.

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