What is different of a complement and a relative

Question

I wonder if there is a difference in meaning between the two sentences that I give

The sentence number one is the one where I consider 1.(shaking) 2.(watching) to be an objective complement.

And the number two is the sentence for which I consider contains a relative, including a non-finite clause forms of relative.

In these two cases, obviously I think there is a grammatical difference, but I'm not sure what native English speakers think about it, besides it looks same with number one when a relative sentence become non-finite clause.

Is really different ? I'm asking if there is a delicate nuance.

1. I felt the floor shaking

2. I felt the floor which is shaking = I felt the floor shaking

3. He may not kiss her in front of us watching

4. He may not kiss her in front of us who is watching = He may not kiss her in front of us watching

Answer 1

There is definitely a difference in meaning.

In sentence 1 the subject can feel the floor perform the action of shaking, while in sentence two the subject can feel (as in touch for example) the floor that is performing the action of the shaking. The second sentence does not imply that you feel the shaking movement of the floor, but you simply feel that the floor is there. In the first sentence you feel the shaking movement.

The difference between sentences 3 and 4 is that in 3 you are saying that he is not allowed to kiss her when us are able to see it, this acts as a sort of condition. If the people are not watching, he is allowed to kiss her. In 4 the us are actually performing the action of watching. Here you are saying that he is not allowed to kiss her in front of us. The who are watching modifies us and does not modify the other part of the sentence. This means that he is not allowed to kiss her, whether those people are watching or not. As long as he is in front of them, he is not allowed to kiss her. However, I think that the fourth sentence may be interpreted in they way you intended it as well, though it seems to me an odd construction for conveying that meaning.

Answer 2

Along the same lines of Sander's answer, but a bit more concise:

1. I felt the floor shaking.
   = I felt that the floor was shaking.

2. I felt the floor which is was shaking.
   = I felt the floor. The floor was shaking.

3. He may not kiss her in front of us watching.
   = He may not kiss her in front of us when we watch.

4. He may not kiss her in front of us who is are watching.
   = He may not kiss her in front of us, and we are watching.