Operating system security questions help

These are the questions on operating system security:

Give an example of the use of physical separation for security in a computing environment.
Give an example of an object whose security level may change during execution.
A directory is also an object to which access should be controlled. Why is it not appropriate to allow users to modify their own directories?
Suggest an efficient scheme for maintaining a per-user protection scheme. That is, the system maintains one directory per user, and that directory lists all the objects to which the user is allowed access. Your design should address the needs of a system with 1000 users, of whom no more than 20 are active at any time. Each user has an average of 200 permitted objects; there are 50,000 total objects in the system.

I would like to have original answers of the above four questions written with a simple language

Thanks,