A centrifuge is a machine that makes use of the centrifugal force to separate the different substances in a mixture. After the process, the lighter substances float on the top, whereas the heavier substances sink to the bottom. Surprisingly, a similar process also happens in English.

To illustrate this, let us consider a group of verbs in English called phrasal verbs. A phrasal verb is formed by joining a normal verb and a preposition. Very often, the compound formed may have a meaning more than or different from its two constituent words. Common phrasal verbs include ‘turn off’, ‘put down’, ‘pay back’, ‘link up’, etc.

Remarkably, a number of these phrasal verbs show certain interesting behaviors. Observe the following sentences:

  1. Please turn it off.
  2. *Please turn off it.
  3. Please turn the TV off.
  4. Please turn off the TV.
  5. *Please turn the TV in your room which is really annoying me off.
  6. Please turn off the TV in your room which is really annoying me.

We can see that the object of a phrasal verb may either occur after the entire phrasal verb or in between the verb and the preposition. However, where the object is is not a random choice. Normally we only consider the sentences 1, 3, 4 and 6 to be natural or grammatical.

You may have noticed that this has to do with the length, of the “weight” of the object.

In the first two sentences, ‘it’ is a pronoun, which can be said to carry less meaning than ‘the TV’ in the next two sentences. ‘The TV’, of course, is much shorter than and carries less information than the phrase “the TV in your room which is really annoying me”. So we would conclude that for phrasal verbs, if the object is very light, such as when it is a pronoun, it can stay right after the verb and before the preposition. However, if it is too heavy, such as in sentence 5, the object cannot withstand the “centrifugal force” in English and is swung to after the entire phrasal verb, resulting in sentence 6. For sentences 3 and 4, the object is neither too light nor too heavy, so both positions are possible.

In fact, this is not a special characteristic of phrasal verbs, but a general phenomenon in English. For example, consider adjectival participles:

  1. This house has a broken window.
  2. This house has a window broken.
  3. *This house has a broken by a fallen tree nearby window.
  4. This house has a window broken by a fallen tree nearby.

It happens even in normal adjectives:

  1. This is an impossible task.
  2. This is a task impossible.
  3. *This is an impossible to achieve task.
  4. This is a task impossible to achieve.

Note that sentence 2 is only acceptable when it is meant for stylistic effects, and is not a common usage.

If you look around more carefully, you can surely find more examples.