A class lasts 40 minutes, and minutes are the unit of time. One hour equals 60 minutes, a quarter of an hour equals 15 minutes, and half an hour equals 30 minutes. Minute unit: minutes. English: minute, which originally means "tiny", also means min (more common in mathematical terms). 、

Millisecond is a relatively small unit of time, which is one thousandth of a second (0.00 1 second), or ms for short. The minimum exposure time of a typical camera is one millisecond.

The microsecond is one millionth of a second (10 minus 6 seconds), or μs for short. During this time, light can travel 300 meters, which is about the length of three football fields, but sound waves at sea level can only travel1/3 mm.

A nanosecond is one billionth of a second (10 minus 9 seconds), or ns for short. Usually used as a unit of memory reading and writing speed.

Picosecond is one thousandth of a nanosecond (10 minus 12 seconds), or ps for short. The fastest transistor runs in picoseconds.