The Physics Journey Of A Cat (Laws Of Motion)

Professor Shonku, the fictional character in some of Satyajit Ray’s short stories, was a scientist who invented many things ahead of their time, in his laboratory in Giridih. He had a cat called Newton, and in this story, a macaw that knew how to speak (who was also featured in one of the original Shonku stories) teaches Newton about the discoveries of his namesake. He describes it in his diary:

22 June

That bird has been coming here every day for a week or two now. He’s quite friendly, but there is something suspicious about him. He cannot have such an extensive knowledge of science without living in a scientist’s home, as I do. It’ll create trouble, but Shonku cannot see it. He is drawn in by its power of speech and its interest in science, to say nothing of its brightly coloured feathers.

I don’t want to say that I dislike it; in fact, I have quite the opposite reaction to him. His knowledge benefits me, because I am still a young cat and do not know much of science yet. The macaw, on the other hand, is at least five years older than I am, and is happy to tell me what he has learnt.

This morning, while Shonku was out for his morning walk, I woke up to find the macaw perched on a chair in front of me. It flew to the back room, and I followed it out to the garden. He identified me as Newton and asked me if I knew why my namesake was famous.

Of course I knew that Newton was a physicist, but I confessed to not knowing exactly why he was famous; what he did that set him apart from the rest.

He told me that Newton discovered the binomial theorem, integral and differential calculus, calculated pi very accurately, and clarified what gravity really was for future use in classical physics, while also discovering his three laws of motion, getting a unit of force named after him, discovering what white light was really made of, and discrediting and disproving plenty of established facts of science. He then asked me what I would like to learn about, in particular.

I only understood about half of what he said, and to a newly minted cat, ‘the binomial theorem’ and ‘integral and differential calculus’ sound quite scary, so I asked about the three laws of motion.

He immediately recited the first one for me, but they were in some sort of ancient language, and so I asked him to speak in a language I understood. He translated them into English for me:

Law I: Every body persists in its state of being at rest or moving uniformly straight forward, except insofar as it is compelled to change its state by force impressed.

I’ll say one thing: people in the seventeenth century certainly had a roundabout way of speaking! However, being both a Holmes and a Newton, I quickly simplified and summarized it.

“Law I,” I said, “means that anything that is not moving, or moving at a constant speed, will stay that way unless a force changes its state of motion.”

He said that that was correct, but he challenged me; he asked that if this law was true, then why did cars slow down after accelerating? Surely if they move at a uniform speed, then they should maintain that speed without continuing to accelerate?

I used my Holmes skills again, and quickly deduced that there must be some external force acting upon the car. I had heard of friction and air resistance, which act on lots of things in the world, and so I announced that it was due to these two forces, which slow down the car.

He told me that I was right, and I asked for the second law.

He muttered a little in that unfamiliar language, then announced the Second Law:

Law II: The alteration of motion is ever proportional to the motive force impress’d; and is made in the direction of the right line in which that force is impress’d.

Now, not even a Holmes could have solved that, so he simplified it:

F = ma

According to the macaw, F was the force applied in Newtons, m was the object’s mass in kg, and a was the acceleration, in metres per second squared (m/s2).

I asked, “What is acceleration?”

He told me that it was a large topic, which he could not cover now unless I wanted to miss breakfast, so he briefly explained that it was the rate of increase in the speed of an object.

We moved on to the Third Law, which was quite easy to understand compared to the other two in its original form, though the concept is more difficult:

Law III: To every action there is an equal and opposite reaction: or the forces of two bodies on each other are always equal and are directed in opposite directions.

No Holmes skills were required here; I just found it difficult to imagine examples from the perspective of a cat.

He told me to imagine that I was perched on the back of a chair, and there was a ledge a few metres away. If I jumped from the chair to the ledge, that would be the action. The reaction would affect the chair; it might make it wobble, or even fall over, provided there was no other force in the equation, such as the force that a person like Shonku or Avinash Babu might exert on the chair by sitting on it.

I had not forgotten the question of acceleration, and I suggested that tomorrow, we could discuss it. He agreed.

Flying above, the macaw saw Shonku returning from his walk. Both of us went back inside for our breakfast.

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