Sudo as Another User

[Alex]

I am logged in as a user without admin privileges. I want to run a 
command which requires admin privs, e.g.

rm /Library/Preference/Logs/panic.log

My understanding of man sudo is that I should be able to run sudo as 
another user with the -u option, thus

sudo -u <admin_user> rm /Library/Preference/Logs/panic.log

However, it doesn't work -- the result is a message stating that the 
current user is not in the sudoers file, etc.

What am I doing wrong? Am I misreading man sudo?

TIA, f