I don't believe it. They're probably measuring mass and velocity and applying the formula afterwards. That doesn't prove anything. Back in Leibniz' days, they would use soft clay and measure the depths of the indentations made by falling weights. This was accurate enough to tell them that the kinetic energy was proportional to mv².
>"proportional to mv²" also means "proportional to 1/2 mv²" and "proportional to 123.45 mv²".
Yes. This is my point. The error has never been found experimentally because everything eventually gets resolved to Newton's E = Fd. Dividing every result by 2 makes no difference because the proportions do not change.
I still don't see why gravitational mass is a problem but I'll keep looking into it.