The resume/CV is a very important part of your application, so you should take care to prepare a strong resume.
For graduate school applications, and in fact, for all things you will apply to, you should always tailor your resume to reflect what the reader is looking for. Most people have one single "master" resume that lists everything, and then have different versions that highlight different aspects of their work depending on what they are applying to. It's even better to go one step further, and instead of just changing which items to show, you should also modify which aspects of each item to highlight.
In your case, especially since you have a mechanical engineering background but are applying to Physics programs, you should ensure that you emphasize all the relevant physics skills when discussing your mechanical engineering work. For example, if you are applying for a lab based physics position, then definitely highlight any lab skills or hands-on work you've done since those are transferable/relevant skills. Other useful skills are any general research skills (reading technical literature, technical writing, presentations, etc.) because these are valuable skills for a graduate student to have, whether it was in physics or not!
Remember, when writing your resume, it's not just about listing your past experience and skills. It's about communicating to the program why you are the right candidate! Think about the qualities that they are looking for so that when you describe your previous experience, you can show how you meet these criteria. This is why everyone should modify their resume depending on what they are applying to!