Buoyancy and its many applications

  • How do large metal ships float in water while small metal coins sink?
  • Buoyancy is dependent on two very simple physical properties, weight and volume of an object. The density of an object is the ratio of weight to volume.
  • Why does hot air rise? Why is it impossible to create a cold air balloon?
  • Positive and negative buoyancy are directly related to the density of an item compared to the density of a liquid. Its is not simply down to weigth!
  • A lifejacket is useless unless inflated, how and why does buoyancy work in this particular scenario?

buoyancy cargo shipLet us take a look at density before we deep dive (or float!) into the science of buoyancy. Density is the ratio of weight of the object to the volume of the object. The density of water is 1g/cm3 and 8g/cm3 for steel. Whenever we place an object on water (or on any fluid), it displaces a certain volume of fluid. To experiment, fill a bucket to the top and then place a wooden block on the water. The wooden block will displace some water and, in our case, the displaced water will spill out. If you collect the displaced water and measure its weight, it will be equal to the weight of the wooden block.

What is Buoyancy?

Buoyancy is the ability of an object to float on water. It depends on two important physical properties, weight and volume of the object. If an object floats on water, it has positive buoyancy and if it sinks, it has negative buoyancy.

So the process is as follows, the moment you place something on a fluid surface, fluid will keep displacing and the object will keep on drowning. As the weight of the displaced water is less than the weight of the object, the object will push more and more water out of the bucket. This will continue until fluid of equal weight to the object has been displaced. If after complete immersion of the object, the weight of the water spilled is less than the weight of the object, the object will drown.

That is, in stable conditions; if the

(weight of the displaced water) = (weight of the object)


then the object will float. If

(weight of the displaced water) < (weight of the object)

the object will drown. Sounds too complicated? Remember, when density (ρ) of the object is lower than the density of water (that is 1g/cm3) the object will float, otherwise, it will sink.

The density of an object plays an important role in determining whether or not it will float. So, if we want to make an object float, we can either change its weight or we can change its volume. So, which of the two properties can we modify? We can’t change the weight of the object, as it is the product of mass (constant) and gravitational acceleration (constant on earth).

Thought experiment: What if we take the bucket full of water and metal block to the moon, will the block float? With the change in gravitational acceleration the weight of the block will change. Let’s discuss in comments!

To make an object float, we can increase the volume which will reduce the density. If it goes below the density of water, it will float. By increasing the volume, the object can now push more water out of the bucket. If the increase in volume is of such an order that the object can push water equal to its weight, then it will float.

Coming back to our questions,

How does a metal ship float?

Ingenious engineers simply increased the volume of the ship’s hull. The increase in hull’s volume is done in such a way that the increase in weight is marginal – the hull will be hollow. So even though the metal (steel, aluminium or titanium) block has higher density than water, the ship is designed such that it has lower density than water. So can you answer the question, why did the titanic sink?

Unfortunately, the iceberg punctured its hull. This led to the escape of trapped air in the hollow hull which was replaced by water. As the water started flooding into the hull, the weight of the ship (including the weight of the water in the hull) started increasing, thereby increasing the density. When the density increased beyond the density of water, it submerged.

buoyancy hot air balloon

How does a hot air balloon work? Why can’t we use “cold air” balloons?

In many ways a hot air balloon is the best way to illustrate buoyancy. The main reason that a hot air balloon works is because the hot air within the balloon is less dense than the cold air on the outside. Therefore, because the balloon is less dense it simply floats. This is the reason why the air is continuously heated and recycled into the balloon. If the air was allowed to cool then the hot air balloon would simply lose buoyancy and lose altitude.

The reason why we don’t have “cold air” balloons is simple. The density of the balloon and the air inside (together with the basket, passengers and equipment) would be greater than the air on the outside of the balloon. Therefore, there is no buoyancy and no lift. In very basic terms, cold air is denser than hot air due to the way in which the molecules are positioned.

How do life jackets save people from drowning?

Just like a ship’s hull, the life jacket traps air inside it and hence, increases the total volume (volume of person wearing it+volume of life jacket) without increasing the weight. This leads to a reduction in density to below that of water – hence it will keep the person afloat.



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