I'm an engineer and I often hear others say "the motor speed" when they are talking about the speed of the motor. For example, one might ask "What was the motor speed?" when he or she wants to know what the speed of the motor was. I've seen similar phrases in scientific journals too. Why is it OK to say "the motor speed" when you are talking about the speed of the motor?

  • 1
    Can you explain why it wouldn't be okay? Do you have the same problem with "internet speed"?
    – Gob Ties
    Mar 27, 2014 at 19:02
  • @Geobits I think the OP may be wondering why we don't say 'the motor's speed'.
    – WS2
    Mar 27, 2014 at 19:11
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    This question is a better fit for ELL. It appears that you are asking about the simple use of an adjective. In motor speed, motor in an adjective, just like it is in motor oil. Mar 28, 2014 at 2:55
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    @WS2 I believe the OP is asking why we place the noun, motor, immediately before another noun, in this case, speed, and we understand it to mean the the speed of the motor. Similar noun adjuncts are a book cover (the cover of a book), a rope bridge (a bridge made of rope), matchbox (a box for matches), a four star hotel (a hotel with four stars) etc.
    – Mari-Lou A
    Mar 28, 2014 at 7:48
  • possible duplicate of Using apostrophes correctly I believe this question may be of valid help. If it isn't, please say so and I will delete my request to close your question as being a duplicate.
    – Mari-Lou A
    Mar 28, 2014 at 7:48

3 Answers 3


If you are wondering why we don't say 'the motor's speed', it is just that idiomatically, with very specific things we simply drop the apostrophe s.

Doctor's and nurses will talk about their patients' 'heart rate', 'pulse rate', 'breathing rate', 'blood count', 'flesh tone'. We talk about having 'kidney trouble', or 'liver trouble'.

When driving a car we will watch out for the 'speedometer reading'. We replace faulty 'headlight bulbs' etc.

In the garden we will use a 'hedge trimmer', 'garden shears' etc.

In none of these examples and scores more, where you might expect to find a possessive apostrophe, they are dropped and a 'compound noun' is formed. Some people might argue that such terms should be hyphenated.

  • How will I know when it is OK to do this? For example, if I'm talking about the age of the universe can I say "the universe age"?
    – Engineer
    Mar 27, 2014 at 19:40
  • @Engineer No I'm afraid you can't say 'the universe age'. It has to be something that is used so frequently that a 'compound noun' has been established. I cannot think of a rule, other than simply learning by listening and experience. I don't know if anyone else can. You may find a good dictionary helpful.
    – WS2
    Mar 27, 2014 at 20:21
  • WS2, you can certainly say "Universe age" in certain contexts - I've used that, funnily enough. No problem at all. (In a context much like "motor speed," we were discussing something like "system age" "ship age" and ultimately "universe age".)
    – Fattie
    Mar 28, 2014 at 10:37
  • "In English, you can use a noun as an adjective." Table age, plastic age, motor speed, TGV speed, light speed, dog years, cat years, internet posts, snail speed, internet speed, USA time, Superhero speed, America speed, Europe speed, racehorse speed, ice age, snail age, internet age, etc etc
    – Fattie
    Mar 28, 2014 at 10:44
  • @JoeBlow Can you answer the OP's question, namely 'How will I know when it is OK to do this?'
    – WS2
    Mar 28, 2014 at 11:21

"Motor speed" is likely used as a contrast to "land speed", "air speed", etc. A motor speed would likely be measured, for example, in RPM, whereas other relevant speeds could be measured in MPH, or some other measure.

For example, in a motor boat, the motor speed is not necessarily proportional to the "actual" speed of the boat (when accelerating, for example, there's some delay between throttling up and actual acceleration).

  • 1
    'Motor speed' is a totally different concept to 'land speed', or 'air speed'. 'Motor speed' is that at which the motor is running. But the others are not the speed at which the land or the air are moving. 'Land speed' is that at which the vehicle is moving over land, and similarly through the air. I feel you have answered the OP as if he had posed an engineering question. But the fact that it is raised on a language site tells me he is wondering why there is no apostrophe s.
    – WS2
    Mar 27, 2014 at 19:38
  • Right. The OP is asking how noun adjuncts work in English.
    – Fattie
    Mar 28, 2014 at 10:38

It's amazing someone hasn't simply answered:

"it's an adjective."

High speed, slow speed, stalling speed, motor speed.

This couldn't be simpler...


one is a "type of thing" (eg, "aircraft door")

the other is possession (eg "John's door")

Full, total explanation...


Full line-by-line explanation of this question:


I'm an engineer and I often hear others say "the motor speed" when they are talking about the speed of the motor.

Wrong. They are talking about the "motor speed" of a specific motor, say TestMotor1234.

For example, one might ask "What was the motor speed?" when he or she wants to know what the speed of the motor was.

Wrong. They are asking "What was the motor speed of TestMotor1234?"

I've seen similar phrases in scientific journals too.

You have not. Sentences need an object.

Why is it OK to say "the motor speed" when you are talking about the speed of the motor?

Why would it not be? "What is the motor speed of TestMotor1234?"

Note that if you physically point at an object and say "What's the color?" that is - of course - the same as saying "What is the color of that object?"

The sentence fragment, on its own "What is the color", is utterly meaningless.

The sentence fragment, on its own "What is the motor speed", is utterly meaningless.

  • Forget about motor speeds for a minute. When does a 'possession' become a 'type of thing', as you describe them? We both agree things like 'barn door, car seat, house paint, tea bag, animal feed, dog tooth, coffee mug etc' don't need apostrophe s. But what about 'men's underwear, lady's shoes,child's necklace, cow's milk, doctor's stethoscope, lawyer's office, etc' - why do they all need apostrophe s? That is what the OP is asking. How can he know when to omit the apostrophe?
    – WS2
    Mar 28, 2014 at 15:56
  • I see the point that Joe Blow is making. However, one would say "the speed of TestMotor1234" and not "the motor speed of TestMotor1234". Speed by itself would be understood as the rotational speed of the shaft. I think the problem might be that I don't understand when the apostrophe s can be left out just as WS2 and Mari-Lou A pointed out. Although, "motor's speed" sounds a bit off too in my opinion. In my native language there exists a similar feature to the apostrophe s and you could not leave it out in this case but in some other cases you could. This is what is confusing me I think.
    – user70400
    Mar 28, 2014 at 18:37
  • A winter coat (a coat for the winter), a raincoat (an impermeable coat / a coat to keep you dry), but we say "a boy's coat" and "women's clothes". The first two compounds describe the type of coat, its use; whereas a child's coat doesn't tell us its function but its owner, and if you visit a department, you'll know that the floor sells clothes made for women (the future owners). Likewise, the dog's tail tells us who the owner is, while the motor speed describes what type of speed, but not its owner. This explanation is greatly simplified, but it should work in many cases.
    – Mari-Lou A
    Mar 28, 2014 at 21:14
  • @Mari-LouA we also say 'cow's milk, a doctor's coat, lawyer's office, men's shoes', and thousands more, yet 'dog basket, water heater, car tyre' etc. There seems no rhyme nor reason as to which one is used.
    – WS2
    Mar 29, 2014 at 6:21
  • "cow milk" "child necklace" etc are perfectly common. when you say "we both agree.." that's wrong. when you say "all need" that's also wrong. when you try to conflate away the difference between a type-of-thing and someone's thing, that's wrong! you're pointing out that you can also (in some cases) use apostrophe-s to refer to a type-of-thing. But so what? Simply re-read the "Full line-by-line explanation of this question:" for full details! no need to copy and paste it here
    – Fattie
    Mar 29, 2014 at 6:55

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