A dunking bird toy, December 28, 2011.
| Photo Credit: RobinLeicester (CC BY-SA 3.0)

Nothing lasts forever. This is good life advice, and it also happens to be an important feature of our physical universe.

Natural philosophers considered and discarded the idea of ‘perpetual motion’ a long time ago. The basic premise of perpetual motion is that it should be possible to operate a system without supplying power to it. We know from daily experience that this can’t be true: for example, your phone’s battery will drop to zero as you use it without charging.

In physicists’ parlance, perpetual motion violates the law of conservation of energy. The first and second laws of thermodynamics also stipulate that anything that offers power must also liberate heat. If there was an infinite power supply, there would also have to be an infinite heat liberator. There isn’t.

But this simple explanation hasn’t stopped some people from wondering whether perpetual motion machines might exist.

A simple example is the dunking bird toy. It uses a temperature differential between the bird’s top and bottom to move back and forth. Very simply speaking, when it moves forth, its beak dips into a glass of water that flows to the bird’s bottom, and the weight causes the bird to move back.

The dunking bird toy can work like this for a long time, but it can’t move perpetually: it will stop when the temperature differential between the bird’s top and bottom vanishes.

Karthik Vinod is an intern with The Hindu.

India’s Jasprit Bumrah celebrates the wicket of Pakistan’s Mohammad Rizwan during the ICC Men’s Cricket World Cup 2023 match in Ahmedabad, October 14, 2023.
| Photo Credit: PTI

India scored a thumping win against Pakistan at the ICC Men’s ODI World Cup match in Ahmedabad on October 14, with the latter’s batters folding for 191 following a collapse at the start of which they were at a healthy 155/2. Five of India’s six bowlers took two wickets apiece in the rout, although they were led by Jasprit Bumrah and Kuldeep Yadav, who induced the collapse with the wickets of Mohammad Rizwan and Iftikhar Ahmed, respectively.

But for all the magic on display, Bumrah’s delivery to dismiss Rizwan in the last ball of the 34th over stands out for illustrating an everyday occurrence in cricket but at a magnitude commonly seen in physics textbooks and high-school laboratories, less so on the cricket field. The delivery was a slow offbreak – a ball delivered at 122 km/hr that, after bouncing, jagged sharply into the right-handed batter. There, a gap between bat and pad, which Bumrah had spent the first five balls of the over opening, allowed the ball to hit the stumps.

While slow offbreaks aren’t exotic in cricket, the one that Bumrah bowled was. After pitching, the ball’s kinetic energy through the air dropped noticeably, so much so that it reached Rizwan much slower than at 122 km/hr. The law of conservation of energy can’t be violated, so where did the ‘missing’ speed energy go?

One possible reason Rizwan was foxed was that he expected the ball to come on to the bat quicker, but it didn’t because of the ‘missing’ energy. When releasing the ball, Bumrah had moved his wrist sideways instead of straight down. As a result, from the moment the ball started moving, it had some ‘forward’ kinetic energy and some angular kinetic energy: i.e. it was spinning.

After pitching, more of the ball’s kinetic energy was converted to the angular component because of the way the ball gripped, and less of it remained for the forward component. The ball would have also lost a little energy as sound and to displace the soil by a small amount when it pitched. The result: it arrived later than Rizwan expected and prised him out. 

To be sure, offbreaks aren’t a novelty, but seldom do we behold one that exemplifies its own features to such a great degree. The messier equations underlying the physics of a shaped object moving through the air in a specific way – like Neeraj Chopra’s javelin or Jasprit Bumrah’s offbreak – often overtake the simpler ones at the core of all physics, but become unmistakeable in moments like 33.6.