Archive for the Cute Problems Category

Oh Larmor! Energy in Electromagnetic Waves

Posted in Cute Problems, The Universe and Stuff with tags , , , on April 16, 2021 by telescoper

This week I started the bit of my Advanced Electromagnetism module that deals with electromagnetic radiation, including deriving the famous Larmor Formula. It reminded me of this little physics riddle, which I thought I’d share again here.

As you all know, electromagnetic radiation consists of oscillating electric and magnetic fields rather like this:


(Graphic stolen from here.) The polarization state of the wave is defined by the direction of the Electric field, in this case vertically upwards.

Now the energy carried by an electromagnetic wave of a given wavelength is proportional to the square of its amplitude, denoted in the Figure by A, so the energy is of the form kA2 in this case with k constant. Two separate electromagnetic waves with the same amplitude and wavelength would thus carry an energy = 2kA2.

But now consider what happens if you superpose two waves in phase, each having the same wavelength, polarization and amplitude to generate a single wave with amplitude 2A. The energy carried now is k(2A)2 = 4kA2, which is twice the value obtained for two separate waves.

Where does the extra energy come from?

Answers through the Comments Box please!

The Mechanics of Nursery Rhymes

Posted in Cute Problems, The Universe and Stuff with tags , , , , on December 30, 2020 by telescoper

I’ve always been fascinated by Nursery Rhymes. Some people think these are little more than nonsense but in fact they are full of interesting historical insights and offer important advice for the time in which they were written. One such story, for example, delivers a stern warning against the consequences of placing sleeping babies in the upper branches of trees during windy weather.

Another important role for nursery rhymes arises in physics education. Here are some examples that students of elementary mechanics may find useful in preparation for their forthcoming examinations.

1. The Grand Old Duke of York marched 10,000 men up to the top of a hill and marched them down again. The average mass of his men is 65 kg and the height of the hill is 500m.

(a) Estimate the total work done in marching the Duke of York’s men up to the top of the hill.

(b) If, instead of marching down again, the men take turns sliding down a frictionless slide back to where they started, estimate the average speed of a man when he reaches the bottom of the hill.

(You may assume without proof that when they were up they were up, and when they were down they were down and, moreover, when they were only half way up they were neither up nor down.)

2. By calculating the combined rest-mass energy of half a pound of tuppenny rice and half a pound of treacle, and assuming a conversion efficiency of 10%, estimate the energy released when the weasel goes pop. (Give your answer in SI units.)

3. The Moon’s orbit around the Earth can be assumed to be a circle of radius r. A cow of mass m is standing on the Earth (which has mass M, and radius R). Derive a formula in terms of r, R, M, m and Newton’s Gravitational Constant G for the energy the cow needs in order to jump over the Moon.

(The Earth, Moon and cow may be assumed spherical. You may neglect air resistance and udder frictional effects. )

Feel free to contribute similar problems through the Comments Box.

A Problem of Resistance

Posted in Cute Problems with tags , on November 29, 2020 by telescoper

Bizarrely, last night I dreamt of this physics problem. This mean that I’ve seen it before somewhere, but if that’s the case then I’ve forgotten where. In the dream the problem of electrical resistance was muddled up with the problem of how to calculate the Euler Characteristic of a structure defined on a grid*, which is something I have used in the past. Anyway, with apologies for the poor quality of the drawing, here is the set up.

Twelve identical resistors R are arranged in four squares with common edges thus:

Yes, they’re meant to be identical squares!

What would be the effective resistance of this circuit measured between A and B?

Please post your answers through the comments box, with appropriate explanations. Bonus marks for elegant (i.e. short) solutions.

(In my dream this problem came up in contrast with the case where the four internal resistors and their connecting wires were absent, so the circuit was just a ring.  The Euler Characteristic of the original connected set of squares is 1 while that of the ring is 0, not that it’s relevant to the problem in hand!)


A Remnant Problem

Posted in Cute Problems, The Universe and Stuff with tags , , , on November 16, 2020 by telescoper


I haven’t posted any physics problems for a while so here’s  a quickie involving dimensional analysis. You have to assume that the supernova remnant mentioned in the question is roughly spherical, like the one shown above (SNR 0500-67.5):

As usual, answers and comments through the box below please!

Click on the `continue reading’ thing if you would like to see my worked solution:

Continue reading


Posted in Cute Problems, Education, The Universe and Stuff with tags , , , , , on August 12, 2020 by telescoper

While this morning’s repeat exams were going on I was leafing through an old second-hand text book, one of many I have acquired over the years looking for nice problems and worked examples. The good thing about old books is that solutions to the problems are usually not available on the internet, unlike modern ones. The book concerned this morning is a classic: Statics by Horace Lamb, which you can still get via Cambridge University Press. I have the first edition, published in 1912.

Looking through I was somewhat alarmed to see what had been pencilled in some of the margins:

Of course anyone who has been to India knows that the swastika isn’t necessarily a Nazi symbol: you find it all over the place in the Indian sub-continent, where it is used as a symbol for good luck. I remember being given a very nice conference bag in Pune many years ago with a swastika on it. I didn’t use it back home, of course.

The first owner of the copy of Statics that I have was acquired in 1913 by a J.H.C (or G) Lindesay of Sidney Sussex College, Cambridge. I know because he/she inscribed their name in the front. That doesn’t look to me like an Indian name, but I think it’s a fair bet that the book passed through many hands before reaching me and that one of the past owners was Indian. I haven’t tried any of the problems marked with the swastika, but perhaps they are difficult – hence the `good luck’ symbol? I notice though that the symbol at the bottom of the page has a chirality different from the others. Is this significant, I wonder?

All of which irrelevance reminded me of an discussion I’ve had with a number of people about whether they like to scribble in the margins of their books, or whether they believe this practice to be a form of sacrilege.

I’ll put my cards on the table  straightaway. I like to annotate my books – especially the technical ones – and some of them have extensive commentaries written in them. I also like to mark up poems that I read; that helps me greatly to understand the structure. I don’t have a problem with scribbling in margins because I think that’s what margins are for. Why else would they be there?

This is a famous example – a page from Newton’s Principia, annotated by Leibniz:


Some of my friends and fellow academics, however, regard such actions as scandalous and seem to think books should be venerated in their pristine state.  Others probably find little use for printed books given the plethora of digitial resources now available online or via Kindles etc so this is not an issue..

I’m interested to see what the divergence of opinions is in with regard to the practice of writing in books, so here’s a poll for you to express your opinion:

The Geostationary Orbit

Posted in Cute Problems, The Universe and Stuff with tags , , , on May 16, 2020 by telescoper

I’m was mucking out this blog’s blocked comments folder and unsurprisingly found a few from Mr Hine, a regular if sadly deranged correspondent.

One of his blocked comments begins

In the forlorn hope that Mr Hine might some day learn something scientifically correct I thought I’d repost this problem, which is very easy if you have a high school education in physics or applied mathematics but no doubt very difficult if you’re Mr Hine.

Verify that the radius of a circular geostationary orbit around the Earth is about 42,000 km, i.e. find the radius of a circular orbit around the Earth which has a period of 24 hours so that its orbital period matches the Earth’s rotation period, thus ensuring that an object travelling in such an orbit in the same direction as the Earth’s rotation is always above the same point on the Earth’s surface.

(You will need to look up the mass of the Earth.)

How to Solve Physics Problems

Posted in Cute Problems, Education, Maynooth, The Universe and Stuff, YouTube with tags , , on May 14, 2020 by telescoper

Since the examination period here at Maynooth University begins tomorrow I thought I would use the opportunity provided by my brand new YouTube channel to present a video version of a post I did a few years ago about how to solve Physics problems. These are intended for the type of problems students might encounter at high school or undergraduate level either in examinations or in homework. I’ve tried to keep the advice as general as possible though so hopefully students in other fields might find this useful too.

A Virus Testing Probability Puzzle

Posted in Cute Problems, mathematics with tags , on April 13, 2020 by telescoper

Here is a topical puzzle for you.

A test is designed to show whether or not a person is carrying a particular virus.

The test has only two possible outcomes, positive or negative.

If the person is carrying the virus the test has a 95% probability of giving a positive result.

If the person is not carrying the virus the test has a 95% probability of giving a negative result.

A given individual, selected at random, is tested and obtains a positive result. What is the probability that they are carrying the virus?

Update 1: the comments so far have correctly established that the answer is not what you might naively think (ie 95%) and that it depends on the fraction of people in the population actually carrying the virus. Suppose this is f. Now what is the answer?

Update 2: OK so we now have the probability for a fixed value of f. Suppose we know nothing about f in advance. Can we still answer the question?

Answers and/or comments through the comments box please.

A Problem of Snooker

Posted in Cute Problems, The Universe and Stuff with tags , , , on February 25, 2020 by telescoper

I came across the following question in a first-year physics examination from Cambridge (Part 1A Natural Sciences) and, since I have posted anything in the Cute Problems folder for a while I thought I would share it here:

Answers through the comments box please! And please show your working!

P.S. The preamble does not say whether you can also assume irrelevant formulae without proof…


Yet another easy physics problem…

Posted in Cute Problems with tags , , , on December 2, 2019 by telescoper

Last week I posted a little physics problem that generated a large amount of traffic (at least by the standards of this blog), so I thought I’d try another one.

The examination comprised two papers in those days (and a practical exam); one paper had long questions, similar to the questions we set in university examinations these days, and the other consisted of short questions in a multiple-choice format. This question is one of the latter. Incientally, for those of you who have asked, the multiple-choice examination contained 50 such questions to be answered in 2½ hours, which is three minutes per question.

(You can assume that the acceleration due to gravity is 9.8 ms-2.)

And here is a poll in which you may select your answer:

Comments on or criticisms of the question are welcome through the comments box…