Yes, the noun 'Americans' is a concrete noun, a word for the people of the United States of America; a word for physical people.
The plural, proper noun 'Americans' is a concrete noun as a word for the people of the United States or the people of the American continents. People are physical beings.
The abstract noun form of the concrete noun 'patriot' is patriotism; a word for an emotion.
The word "All-American" as a noun is a proper noun referring to a person, an athlete. So it is a concrete noun.
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The possessive form of the plural noun Americans is Americans'.Example: Many Americans' lifestyles are harmful to the environment.
The plural, proper noun 'Americans' is a concrete noun as a word for the people of the United States or the people of the American continents. People are physical beings.
The proper noun Phoenix (city in Arizona) is a concrete noun. The common noun phoenix would be a mythological creature, which if encountered would also be a concrete noun.
Concrete. (You can see it, feel it, bite it!)
No, Americans is not a collective noun. The collective noun that would represent Americans as a group would be country, nation, people, electorate, etc...
Concrete. It refers to something physical that you can reference directly. Additionally, it's a proper noun, which denotes the name of a place. An abstract noun would be "love" or "sickness" which you can't point to. A concrete, but common, noun would be "planet" instead of Mars.
The noun 'cafeteria' is a concrete noun as a word for a physical place.
Door to success is an abstract noun. It depends
The noun 'authority' is an abstract noun, a word for a concept or idea. There is not corresponding noun for authority; a concrete noun would have to be the specific person that has authority. For example, the principal of the school has authority, but 'principal' is also a concept, not the person. So the concrete noun would be Ms. Washington the Principal, or perhaps Police Chief Wiggam.
Police would be a concrete noun. You can see police, you can (though I wouldn't recommend it) touch police, you can also hear police. If you are able to use your senses to identify the noun, then it is concrete.
Proper Noun
The noun 'Philadelphia' is a concrete noun, a word for a physical place.
Concrete. (But few bathtubs are made out of concrete.)