The boat has a special shape that is designed to push water aside. The more water that is pushed aside the more force that the water exerts upwards. For example if the boat pushes aside 300 kilograms of water then the water exerts a force upwards on the boat able to support 300 kilograms.

Pushing water to one side is called displacement of water. Giant cargo vessels may weigh thousands of tons but they displace thousand of tons of water, which is why they do not sink.

What keeps both the boat and the hot air balloon afloat?
There is a force keeping these from sinking or falling from the sky.
We always think of buoyancy as relating to sea vessels but hot air balloons also use this force to lift off the ground.




Buoyancy is an upward force exerted by a fluid, such as water or air, that opposes the weight of an immersed object. This force increases as the amount of displaced fluid increases. Balloons are objects immersed in a fluid, cold air, and displace this fluid by inflating. Hot air is pumped into the balloon pushing the walls outward to inflate the balloon.

As the balloon inflates it displaces the fluid it is immersed in, in this case the fluid is air.

The weight of this fluid, cold air, is the force (buoyant force) that pushes the balloon upwards against gravity.

A cubic metre of air at sea level has a mass of 1.2 kg. So any object that has a volume of 1 cubic metre is buoyed up by a force able to support 1.2 kg. If the mass of the object is more than 1.2 kg it will fall to the ground. If a one cubic metre object has a mass less than 1.2 kg it will rise skyward.


1) Design an experiment to test the hypothesis that salt water exerts more of a buoyant force than freshwater.

2) A fish and a hot air balloon both hover at a given level for the same reason.

What is the reason for this?



3) A helium filled balloon has a mass of 20kg and a volume of 16 cubic metres. It is released outdoors at sea level when the temperature is 20oC . Will the balloon rise or stay grounded? Explain.


4) The density of air at 0oC is 1.29 kg/m3 while at 30oC it is 1.16 kg/m3. Use this information to explain why we see hot air balloons floating in the sky during cold winter mornings rather than on hot summer days.


5) The density of hydrogen is 0.09 kg/m3. Two balloons are inflated to the same size, one filled with air and the other filled with hydrogen. Which balloon experiences the greatest buoyant force?


6) A ten kilogram brick is lowered into a tub of water. A volume equal to two litres of water is displaced.

If the density of water is 1g/cm3 what is the reading of the scale when the brick is fully submerged?

Does the brick lose mass when it is placed in water?


Make your own toy boat- the rice challenge
Activity to measure the buoyant force.

Put-Put boats and buoyancy

Another activity in determining the buoyant force