On the density of milk and weightlessness in space

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

16 February 2025

Jones Wafula saw a newspaper story which said that “a typical Kenyan consumes 86.1 kilograms (86.1 litres) of milk each year.” He wondered about this statement because it implies that milk has the same density as water – one kilogram per litre. He adds, “I'd have expected milk to be denser than water given that milk ought to have additional matter inside it.”

This is true: milk is denser than water. One litre of this liquid, is about 1.03kg. Thus, 86.1kg of milk would be 85.6 litres. On the other hand, 86.1L would be 88.7kg. I have not seen the full newspaper story so I do not know the original source of the numbers, but they are very precise so they need to be equally accurate.

This is where communication of numbers becomes an artform. I am quite sure that anyone who reads “86.1kg” actually only registers 86kg. The third digit is lost/ignored. It is for this reason that businesspeople price their goods at, say, Sh499 instead of Sh500. The reader’s mind only registers the “four hundred” part of the number. It is not until you present a Sh500 note and get one shilling back that you fully appreciate the true cost of the product!

Going back to the milk story, it is common for people to assume that one litre of anything is equal to one kilogram in mass. Take cooking oil, for example. Many assume that 5L is 5kg. But the density of oil is about 0.9kg per litre. Thus, 5L should be about 4.5kg.

The keen reader will have noticed that I am not referring to these things “weighing so many kilograms”. The reason is that weight and mass are different: weight is the force exerted by a substance due to the pull of gravity while mass is a measure of the quantity of matter (all the electrons, protons and neutrons).

So; the weight of a substance depends on the strength of gravity at its location. Its mass, on the other hand, does not change. When astronauts experience weightlessness in orbit, they are not massless!

Finally, this is also a good point to dispel a common misconception: that astronauts at the International Space Station are weightless because there is no gravity. This is not true! There is quite a lot of gravity at that height – about 90 percent of the value at the earth’s surface. The weightless is due to the orbital motion of the ISS around the earth. The force due to this motion cancels out that from gravity.