Dropping out of grad school - worth getting MS?
I suspect the statistic is still valid that 1/2 of all new businesses fail in the first 5 years. You're worried about a grade in a class, but think about a failure that costs you your life savings, credit, etc. As a physicist, you probably have zero training in business, management, HR, finance, etc., so you can pretty much plan in being in the downside of the 1/2. Stick to what you know.
Time to be blunt. Grow up. Drop the class you are failing (I'd still audit it), and pick it up next year, unless, you are failing because you are clueless. If that's the case, find another profession you can do well. Assuming you didn't barely get by getting your undergraduate degree, you're probably not clueless.
So look at motivation. Do you lack it because you are concerned about getting a job? Perhaps you don't want to be a lab rat the rest of your life. The extra 1-2 yrs to get your MS is worth it. The fact is, at the BS level, you are one of many. As the degree goes up to MS or PhD, there are fewer and fewer, which ultimately means a higher probability of get the select positions when they open up in a company. The degree makes a major difference.
Closing on a nicer note. Check out physics jobs in the major Society journals. Not just lab rat jobs. Physicists work in the oil industry doing well logging, medical physics doing clinical imaging and cancer care, nuclear physics in industry or power plants, national labs (Hanford, ANL, ORNL, LANL, etc.), etc. There truly is a lot out there. Open your eyes, look, find something that trips your trigger, and go for it.