"No work is done by NRF ___ weight." Which conjunction (or/and) appropriate in this context?

Question

The General Question

Which of the following is grammatically correct?

1. It's not good for him or her.

2. It's not good for him and her.

Using neither..nor or either...or structure is a very safe alternative:

3. It's good for neither A nor B.

4. It's not good for either A or B.

But sentences 3 and 4 are too stilted for casual day-to-day speaking purposes.

For reasons not obvious to me, sentence 1 sounds more "correct". Maybe because the conjunction or is also used in sentences 3 and 4, which are very familiar structures.

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Specific Context-based Question

Which conjuction should I use in this case?

NRF of 200 N is acting upwards and weight of 200 N is acting downwards. But the object is not moving in either of these two directions. Therefore, no work is done by NRF ___ weight.

Answer 1

Sentences 1 and two are both grammatically correct: you would use sentence 1 if you wanted to talk about the effect on the two people individually, and sentence 2 if you wanted to talk about the effect on them as a couple.

As you say, 3 and 4 are somewhat more formal, and also more emphatic.

Note that, unless you particularly want to emphasise the not, you would be much more likely to say it isn't, rather than it's not.

Informally you would probably say

It isn't good for either of them

Regarding the text that you have added: it's important to be specific as it is explaining a scientific point, and it's OK to be formal in this context. I therefore think that either... or or neither ... nor would be appropriate:

An NRF of 200 N is acting upwards and a weight of 200 N is acting downwards. The object is not moving in either direction, therefore no work is done by either the NRF or the weight.

An NRF of 200 N is acting upwards and a weight of 200 N is acting downwards. The object is not moving in either direction, therefore neither the NRF nor the weight are doing any work.

Note that indefinite articles are required in the first sentence (because we are describing the two new forces for the first time), and definite articles are required in the second sentence (because we are referring to forces that we have already described).

Answer 2

Neither...nor is not "too stilted for causal day-to-day speaking purposes". That is an exaggeration. You could say to a car salesman, for example, after he had presented you with two options:

Neither the X nor the Y gets good enough gas mileage for me.

Although you might not begin the conversation with such as sentence, as that would indeed be somewhat stilted. It would certainly tell the car salesman that you were a "no-nonsense" consumer, not one to be bedazzled by shiny chrome.

Or you could say to a friend

Neither the red blouse nor the green blouse looked good with that skirt. I liked the blue one best.

if the conversation had progressed something like this:

Did you like the red blouse?
--Not really.
How about the green one?
--It clashed with the skirt. In fact, neither the red blouse nor the green blouse went very well with that skirt. The blue one looked the best.

You could say, as JavaLatte has suggested, "It isn't good for either of them." Or you could say "It's good for neither of them."