I work for a company that hired me at 40 hours a week (as well as many other). They make up new rules each day as to why they are sending us home. There was no mention of any of this when hired that this could occur.
I did visit with HR concerning this and they said our work is based on need.
They have hired so many people and have two more classes in training now that will also go to the PM shift. What do you think about this?

Unless you have some sort of contract that guarantees 40 hours there little you can do. You might take the extra time you now have and seek more relible work with better hours

Unless you have an enforceable contract that guaranteees you forty hours a week, you are out of luck. I would not be surprised if this company has high turnover. Go figure.

i do have paperwork that states 40 hours a week and full time....will that work?

Probably not - I doubt that a paper would rise to the level of an enforceable contract. However, you can hold for other opinions.

A piece of paper is not a contract.

That paper undoubtedly explains what the employer's standard work week is. It is not a guarantee that you will work those hours and only those hours.