Fast vs. rapid

EChong asked, "When should fast be used, and when should rapid be used?"

Thank you for asking this interesting question. Fast and rapid are synonyms that both mean "moving quickly or happening quickly." But, as with many synonyms, there are differences between these words, and these differences can help you decide when to use each of them:

1. Fast can be an adverb as well as an adjective. It is sometimes used as a verb, too, with a completely different meaning. See the examples below.

• She's a fast runner. (Fast is an adjective describing runner.)

• You're driving too fast! (Fast is an adverb describing driving.)

• The prisoners fasted to protest their living conditions. (Fast is a verb that means "to eat no food for a period of time.")

The most common nouns that fast occurs with are food, track, lane, and pace.

2. Rapid can be an adjective, but it cannot be an adverb or verb. Here are some examples of sentences with rapid:

• There has been a rapid growth in the number of new businesses in town.

•  The patient presented with a rapid heartbeat.

The most common noun that rapid occurs with are growth and change.

3. As you may notice from the examples above, rapid is a more formal word than fast, and it is used much more commonly than fast in academic language, as shown in this example from the journal Bioscience (Feb 2011, Vol 61):

• "It's very likely that a swing in the Pacidifc Decadal Oscillation, which can exhibit rapid changes in sea surface temperatures that last for several years ina row, was under way. "

To sum up, use fast in more casual contexts, such as speaking or email messages to friends/family and especially in the expressions fast food, fast track, fast lane, and fast pace.

Use rapid in more formal language, especially in academic writing, and especially in the expressions rapid growth, rapid economic growth, and rapid change.

I hope this helps.