Marcus Wilson

Dr Marcus Wilson is a lecturer in the Engineering Department at Waikato University and author of the Physics Stop blog. His current research involves modelling of the electrical behaviour of the human brain during natural sleep, focussing particularly on the transitions between sleep states. Previous research interests include infra-red physics and signature control (stealth) and quantum Monte Carlo methods. He graduated from Cambridge University in 1992 (BA Hons) and completed his PhD at Bristol University in 1995.

Repairing a hole in the wall with the diffusion equation - Physics Stop

Nov 20, 2020

Last week child number 1 managed to put a hole in a plasterboard (‘gib’) wall. I wasn’t in the room at the time, but I believe it had something to do starting at one end of the house,  running down the corridor, through the bedroom doorway, doing a forward flip onto the bed and sliding feet first into the wall at the end of the other end of the house. Unfortunately, the house isn’t built for gymnastic floor routines. So we’ve been repairing the wall. Actually, it’s not that difficult to repair a hole in plasterboard, so long as it is only a few centimetres across.  Secure a bit of mesh into the hole (get that at the DIY shop or make your own out of wire) and build up layers of wall filler. Then sand it back to be … Read More

Fancy coughing at your phone? Nicer than a swab up your nose - Physics Stop

Nov 03, 2020

The pandemic has certainly been a trigger for some really interesting science to be done. Here’s something that’s hit the headlines today – Artificial Intelligence can be used to detect COVID-19 from the sound of someone’s cough. If you want to read it, the full report is here, but unless you’re lucky enough to subscribe to IEEE you’ll only be able to read the abstract. A game-changer in the fight against COVID-19? Hard to say right now, but it certainly has potential. The sensitivity of the algorithm is impressive: 98.5% overall, and 100% for people who don’t have symptoms. That means there’s the potential for nearly everyone who has COVID-19 to be identified. One can easily imagine the potential of this. Perhaps not so much for New Zealand*, but in a country where a great many people have … Read More

Scholarship Physics - Physics Stop

Oct 16, 2020

It’s that time of year when school students become seriously focused on exams. This year has been messy for student learning, and has affected some students more than others, but the NCEA external assessments and the Scholarship exams are going ahead pretty-much as normal. I’ve taken some interest in the Scholarship Physics exams over the past years. It’s an exam that’s intended to flush out the very best physics-thinkers in the country, and award them appropriately. A student who does well at scholarship physics is one who doesn’t just know physics, but can ‘think’ physics. They can recognize physics principles and apply them in unfamiliar situations and talk critically about what they are doing. In other words, a student who gets Scholarship Physics is demonstrating what a real physicist does. And that’s why I like the exam. Read More

When analogies are taken too far: Spacetime is bent, but it’s not quite a stretchy membrane - Physics Stop

Sep 30, 2020

Last week I was asked by some school students about the nature of gravity. What is it? Isaac Newton, and a whole pile of textbooks following him, treat gravity as an attractive force between two objects. It’s a force that is proportional to the product of the masses of the two objects, and is inversely proportional to the distance between the objects squared. That is, F = G M m / r^2, where M and m are the masses of the two objects and r is the distance between them. The constant G, known as Newton’s Constant of Gravitation (or ‘big G’, for short), is given by  6.6741 x 10-11 m3 kg-1 s-2, with an uncertainty of about 5 in that last digit. ‘Big G’ is really hard to measure well, and its value is far more uncertain than … Read More

Friction and the Anti-lock Braking System - Physics Stop

Sep 22, 2020

Yesterday afternoon I had to call on my car’s anti-lock braking system (ABS). For reasons best known to its driver, a car pulled out of a side road right in front of me while I was driving home after work, and I needed to stop in a hurry. I rather think the car behind me had to rely on its ABS too, to avoid hitting me. I don’t think I’ve ever had to brake that hard in this particular car before. The crunching noise the ABS makes and the vibration in the brake pedal certainly took me by surprise, even though I knew that’s what it does. The ABS allows the brakes to operate to maximum effect by controlling the braking so that the wheels are just at the point of slipping, but not beyond. The video shows the … Read More

Post-communist Bulgaria, solar eclipses, and NZ Level 2 restrictions - Physics Stop

Sep 01, 2020

In August 1999, I had the opportunity to travel from the U.K. to Varna, Bulgaria, on the Black Sea coast, to view the European Total Solar Eclipse. Just why we ended up in Varna, as opposed to a plethora of other possibilities in Europe, is a bit of a long story, but a substantial part of it was a near guarantee of cloud-free, Black Sea summer. The eclipse itself was fantastic. Unfortunately, unless you are very lucky, you will need to travel to see a total solar eclipse. That said, New Zealand, which is currently in a bit of a solar eclipse drought (there’s a tiddly partial visible in the south of the South Island at the end of next year), will experience monsoon eclipse conditions in 2028. In the subsequent 17 years, it will get three total … Read More

Bad light stopped play - Physics Stop

Aug 26, 2020

One of the top challenges for physics in the modern era, along with Climate Change and explaining Dark Energy, has to be fixing the problem of bad light*. (I’m talking cricket – what else?) It’s a quintessentially English problem. It’s not raining, the pitch is perfectly playable, the spectators (COVID-19 notwithstanding) are enjoying themselves, but the umpires bring the teams off because they deem the conditions unplayable. So the second test between England and Pakistan manages only a day’s play out of five – partly because of rain but also because of Bad Light. It’s frustrating, but at a professional level it really is a safety issue as much as one of fairness. I’ve done my share of amateur 20-over-a-side cricket games squashed into that short period between finishing work and sunset, and, late in summer particularly, the last … Read More

A battery charge meter that actually works - Physics Stop

Aug 11, 2020

If you drive an electric car more than trivial distances between charges, you likely appreciate a state-of-charge meter (that is, what would be called a fuel-gauge in a petrol car) that is accurate. When it reports a range of 30 km, you do want to be sure it will actually do this distance. If you run out of fuel in a petrol car, you can get a lift to the nearest petrol station, fill up a can of petrol, return to your car and get going again. With an electric car you need towing. But there are numerous problems. How quickly your battery discharges will clearly depend on how you drive the car. An estimate of range will make some assumptions about how you are driving, and in what kind of conditions. That’s true in a petrol car too. Better … Read More

The Beirut explosion shockwave - Physics Stop

Aug 06, 2020

That was clearly a huge explosion. Just after the explosion, we see a cloud of ‘fog’ moving outwards at high speed. This is a shockwave, rather similar to that which causes a sonic boom. The ‘fog’ is caused by water condensing from the atmosphere in areas of intense low pressure and temperature, and parallels the sonic boom cloud, or vapour cone, such as in this photo. The shockwave consists of a front of extremely high pressure, caused by compression (squashing) of the air, which is followed by a region of very low pressure where the air has been stretched. When air is rapidly expanded, it cools. Feel the nozzle of a compressed aerosol spray next time you use one. It will be cold. The resulting cold, low pressure air cannot hold as much water vapour and … Read More

All-pervading Waikato dampness - Physics Stop

Jul 17, 2020

Yesterday we arrived back in Cambridge after a few days holiday in Auckland, being tourists. We sampled such delights as the unheated hotel swimming pool,  the complicated and expensive process of getting on a bus (basically having to find somewhere from which to buy a HOP card, for a non-refundable $10 a card), the completely non-social distanced pedestrian rugby scrum while navigating the building site outside Britomart, and the incessant foggy-drizzle of Tuesday that rendered the beautiful view of the city from Devonport utterly invisible. Returning on the now-open Huntly bypass gave me a view of some Waikato countryside that I hadn’t seen before. And passing wetland-after-wetland in the pouring rain reminded me that much of this wonderful region of Aotearoa is swamp. In winter, one cannot escape the fact that Waikato is damp. We then face the usual dilemma … Read More