I have researched this question considerably over the years. This is what I have found: 1. Nearly every state has a list of legal definitions. 2. Many states do not consider a towed motor vehicle to be a "trailer". 3. Many states do not require a braking system on a towed motor vehicle. 4. Nearly every state has a Braking Performance Law. You MUST be able to stop your combination of vehicles on a hard dry clean surface within a certain distance from a certain speed. Here in Montana that law is MCA 61-9-312. It is also in the Federal Motor Vehicle Safety Standards (FMVSS). 5. At 60 MPH, if a kid or dog or deer runs in front of you it is entirely likely that you will not get your foot on the brake pedal before you hit whatever it is. IIRC, you will travel something like 168 feet before you begin to stop! So, the idea of making an emergency stop is a moot point!
So, there is very likely no legal requirement for brakes on your towed vehicle IF you can meet the braking performance standards. AFAIK, nobody has ever tested braking systems to see if THEY will meet the Braking Performance Standard. In fact, I have never read ANY tests of braking systems by independent testing people (Consumer Reports, Good Housekeeping, etc.)
As for physics, lawsuits, etc. I have no idea. I have never read of any such lawsuit, but that doesn't mean they haven't happened.
So, the bottom line is, IF you can meet the Braking Performance Standards promulgated in YOUR state laws, you probably have no legal requirement to have brakes on your toad. Morality is entirely up to you!