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 buoyancyIn the third century BC, a

Greek mathematician named Archimedes realized the relationship between displaced fluid and the buoyant force

Archimedes’ principle states : The buoyant force on an object is equal to the weight of the fluid displaced.

So a rock that displaces 9.8 N of water will have a buoyant force of 9.8 N and weigh

9.8 N less while under water.

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.

Why objects Sink or Float?

What will happen if the buoyant force on an object is equal to its weight? It Floats!!

What will happen if the buoyant force on an object is less than its weight? It Sinks!!

What will happen if the buoyant force on an object is MORE than its weight? It will pop up, like a beach ball when

pressed under water. It only needs to displace its weight!!!

12.3 Density and buoyancy

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

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

This means a full ship must displace more water (sink deeper) to make the buoyant force large enough to balance the ship’s weight.