There is no law, that I know of, that says an employer cannot require you to work more then 40 hours a week, as long as they are paying you the correct wage / hours and are not violating laws (like you can only operate a vehicle for X hours a day etc)

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You can be fired for it as well, as you can be fired for any reason that is not a protected one

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however you will most likely win unemployment if you are fired for being required to work outside your agreed to schedule