Density and Buoyancy

Let’s think...1. When we put the candle in the

water in floated. Why did this happen? Explain in your notebook.

2. What happens when we put it in rubbing alcohol instead? Observe and then write a possible explanation for what you see.

Let’s think...1. Now let’s observe the density column. What do you

notice? How would you explain this?

Rule writing...1. Write a rule that

covers ALL of the observations we have made so far regarding floating and sinking.

Rules for floating and sinking...● When a substance is less dense than the fluid* it is

in, the substance will float.

● When a substance is more dense than the fluid* it is in, the substance will sink.

*A fluid is a liquid OR a gas.

Examples of when these rules hold true include: ● A boat floating in a water while an anchor

sinks. ● Vinegar sinking in oil while rubbing

alcohol floats on water. ● A balloon filled with helium floating in

air.

So why does this happen? ● There must be forces involved in this process. ● What is a force? A force is a push or a pull. ● What force do we KNOW is acting on all objects or

substances on earth?

Gravity! ● What other type of force, besides gravity, must

exist if objects can float in a fluid?

So what is buoyancy?● Buoyancy (also known as the buoyant

force) is the upward force exerted on a substance/object that is in a fluid.

● Gravity acts downwards and buoyancy acts upward!

Understanding Buoyant Force1. What must be true about the buoyant

force and the force of gravity if the object sinks?

2. What must be true about the buoyant force and the force of gravity if the object floats?

Understanding Buoyant Force

1. What would happen if buoyant force and gravity are equal?

This is called neutral buoyancy...

Archimedes PrincipleBuoyant force is equal to the weight of the volume of the fluid displaced by the object. This is known as Archimedes Principle.