I am currently a Mechanical engineering major and my school offers a double majors in both physics and math. I am confused on if i should do it. I like engineering but i also really like physics and math. I am currently a freshman and was wondering whether a double major is a good idea.
Usually a double major is pretty useless. I honestly.cant think of a scenario where it would be truly beneficial.
Yep - typically not useful with an Engineering major. There are many threads on this topic with more thoughts / details on double majors and minors.
Note - I did a a Technical Communications concentration to go along with my Mech Eng degree, but it was pretty easy to do (5 courses, some of them that also filled required elective slots).
@boneh3ad I can think of one scenario where it may be useful. UCF has an engineering and biomedical sciences double major program, which can help if you are pursuing a career in engineering devices for use in the health industry.
@blueman16 Pretty sure that ME/Bio Science program is targeted at Pre-meds (UCF doesn’t offer a Biomedical Engineering BS) and it requires a crazy, silly, number of credits (163). When you take into account that most UCF students enrolling into the program would have 30+ credits from AP/IB/AICE, etc, it may not be too crazy…but still, that’s a lot additional courses. 
http://today.ucf.edu/ucf-begins-double-degree-engineering-biomedical-sciences-pre-med/
To the original question, if you find the additional Physics/Math classes of interest, and it will not add a significant amount of time to graduate (which adds significant cost), then go for it. However, also take into account how much of your time would need to be dedicated to the 2nd degree, that could have been spent on a design team, doing undergraduate research, joining clubs (and taking on leadership responsibilities), etc.
and/or taking deeper or broader engineering courses.
As has been said, anything you say yes to means saying no to something else. Choose wisely.