Matter

Chapter Twelve: The Physical Properties of Matter

• 12.1 Density

• 12.2 Buoyancy

• 12.3 Properties of Materials

Investigation 12B

• Steel is more dense than water so why do steel boats float?

Sink or Float

12.2 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 weight of this rock?

What is the rock’s mass?

12.2 Weight and buoyancy

• 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.

12.2 Weight 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?

12.2 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.

12.2 Weight and buoyancy• What is the buoyant

force on a rock with a volume of 1,000 cm3?

• In air, the scale shows the rock’s weight as 29.4 newtons.

• When the rock is completely submerged, the scale reads 19.6 newtons.

• The difference is a force of 9.8 newtons.

12.2 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?

12.2 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.

12.2 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?

12.2 Density and buoyancy• Average density determines 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.

What can you say about the average density of these blocks?

12.2 Boats and average density• Use your understanding of average

density to explain how steel boats can be made to float.

12.2 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.

• A full ship has more mass than an empty ship. • This means a full ship must displace more water

(sink deeper) to make the buoyant force large enough to balance the ship’s weight.