Most people talk about how much their baby weighs, and while Christopher now weighs about 8.1kg, that is not his true mass. To understand this I need to explaining inertia.
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Newton's first law of inertia states that objects tend to resist changes in their state of motion. An object in motion will tend to stay in motion and an object at rest will tend to stay at rest unless acted upon by a force."
In other words inertia is an indication of the amount of effort required to bring about change, as velocity (change) is a product of force (effort) and mass we can calculate the mass by working out the effort required to bring about change. Therefore to measure the true mass of the baby we need to look at the effort required to bring about a change.
In order to do this compare two couples, one without a baby (couple A) and one with (couple B), the only difference being the little bouncing bundle of joy. Therefore any additional effort required would be as a direct result of the baby, from this we can the determine the mass of the baby.
Picture this scenario, both couples are going to a braai at a friend's place.
Twenty minutes before they have to be there Couple A grab their wallets and jackets, hop into the car and head off. On the way they stop at a supermarket to get meat and salads and pop into a bottle store to get a couple of drinks.
Now Couple B; the night before they work out what time they need to start the first feed in order to ensure the lunch time feeds can slot in with the braai. A feed just before they have to leave and they'll be late, a feed too early and junior will be niggly at the braai and may require extra feeds while they are out.
Next they need to plan the how many bottles they are going to need, what toys need to be taken, will a camp-cot be necessary, do they need to take sleeping blankets, etc. Once all packed it is then getting parents dressed (because clothes are covered in drool/butternut/etc) and then get junior dressed. Once everyone is dressed then junior is put into the car seat and all paraphernalia is put in the car. Stage 1 complete.
Now drinks and food. First find a supermarket with sufficient parking, pram out, baby in pram, everyone into the shops, buy food and then back to the car. Baby into car seat, pram into boot. Next find a bottle store, repeat the same procedure again.
So far Couple B had to start preparations a couple of hours before Couple A.
Now tragedy strikes and the friend's house burns down.
What is Couple A's reaction? "Jeez, really sorry, is there anything we can do?", and the reaction of Couple B: "Sorry to hear about your house, but we're on our way already, at least the coals will be ready!"
It is not that Couple B doesn't care it is just a case that Couple B has too much inertia now to be able to stop, they have to go through with the braai.
I used to sail competitively on a 65 foot yacht called Merlin. The mainsail of Merlin weighs about 100kg, and I would compare effort of getting ready to go out with Christopher to raising the mainsail about 5 times. So if you ask me what Christopher weighs, it is 8.1kg. If you ask me what his mass is, it is about half a ton, and that is a LOT of inertia to get moving and to stop once set in motion!