Page 117, sidebar:
This is still not quite right... The main difference between European and North American mains electricity is the voltage [amplitude], which is 220 VAC rather than 110 VAC. The converters that are used change the voltage, very rarely the frequency; and electronic equipment (which converts electricity to low-voltage direct current before using it) generally does just fine once the voltage is adjusted.

TVs and VCRs will be unusable because the standards (lines, scan frequency, etc) differ as well -- Europe uses SECAM or PAL, North America NTSC -- but only for that reason. Big cities actually have a niche rental market for ethnic videos to run on imported machines!

The only common device that will absolutely not work even with voltage conversion is the electric clock, which will run fast by 1/5 if taken West across the Atlantic or slow by 1/6 if taken the other way. This would include timers on stoves, washing machines, etc, but nobody would try to convert that many amps!

Other motors will also run at the wrong speed, but this will usually not matter too much. North American motors will run slightly hot and slow in Europe, as the reactance is less; but this will not usually be a problem. European motors generally run quite well at 60 Hz.

By the way, some parts of Canada used - until the 1950's, when the continental grid was completed - to use 50Hz 110V power. Or so I've heard; I don't have a source.

Finally, students are studying amplitude as well as frequency in this chapter, so mentioning it would be a good idea! May I suggest, for the next edition, something like:

The electricity supplied to homes in some other countries differs both in voltage (amplitude) and frequency from that in North America. In the United Kingdom, for instance, the line voltage is 220 volts and the frequency is 50 cycles per second. The higher voltage abroad would destroy most North American appliances.

Travellers who bring electrical equipment with them can use a transformer to get the voltage that they need, but the frequency does not change. Most electronic equipment and heating elements will work equally well at both frequencies, but many North American motors will run too slowly and too hot on 50 cycle power, which can cause problems.

Page 126:
Doubling the period of the trapeze (as done in the graph in the revision) quadruples its length, to around 4m [it's really more complicated than that, but if we are going to assume simple harmonic motion then we are already using the small-oscillation approximation]. But that is still not quite enough for Maria to be able to move 10m back and forth! Lower on the page the period is implied to be 8 seconds rather than 4; this would correspond to about a 16m trapeze, which works perfectly. Maria would only be swinging about 40 degrees from the vertical, so the small-oscillation approximation would be usable. (The length is not unreasonable, either, for a troupe like the Cirque de Soleil that tends to the spectacular.) Relabelling the graph (redoubling the labels) thus solves the problem. By the way, it might be nice to have a sidebar in the next edition, pointing out that the motion of a pendulum (such as a trapeze) is not a perfect sinusoid, but that for reasonably small swings it's an excellent approximation; and that it is a general fact that small enough oscillations of almost any system are approximated by sinusoidal functions.