PROPERTIES OF MATTER 12.3

Chapter Twelve: Properties of Matter

12.1 Properties of Solids

12.2 Properties of Fluids

12.3 Buoyancy

Chapter 12.3 Learning Goals

Define buoyancy.

Explain the relationship between density and buoyancy.

Discuss applications of Archimedes’ principle.

Investigation 12B

Key Question:Can you make a clay boat?

Buoyancy of Fluids

12.3 Buoyancy is a forceBuoyancy 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.

12.3 Volume and buoyancyThe 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?

12.3 Weight and buoyancyWeight 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?

12.3 Weight and buoyancyIn 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.

12.3 Weight and buoyancyIn 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.

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?

12.3 Weight and buoyancyBuoyancy explains why some objects sink and others float.

Whether an object sinks or floats depends on how the buoyant force compares with the weight.

12.3 Density and buoyancyIf 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?

12.3 Density and buoyancyAverage 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.

12.3 Density and buoyancy

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

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

However, the steel ball has more weight since it has a higher density.

12.3 Boats and average densityUse your understanding of

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

12.3 Boats and average densityIf 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.

Investigation 12C

Key Question:What is the maximum load a boat can hold

before sinking?How is the maximum load affected by the

density of the water in which the boat floats?

Density of Fluids

The HullThere are many

different types of boats, but all have one thing in common—the hull.

The hull is the main body of the boat. It displaces the water that provides the upward buoyant force. It also provides stability.