Despite pretending to know what he was talking about when telling the Mythbusters what they got wrong when testing Archimedes's "death ray", Obama clearly knows nothing about history or science. (And has apparently never seen any of the many films set in or around ancient Greece.)
Greek shields were concave, meaning they acted as a parabola and reflected the light in one concentrate beam from the centre. Insisting that the problem with the experiment was the number of people holding flat mirrors was historically and scientifically ridiculous.
A punch is not the same as dropping a weight with the same force.
Hellboy's arm didn't fall off and land on the floor, it was an immovable object that struck the car and continued applying a downward force.
It also struck the very front of the car, (the film clip showed the front bumper folding round his hand) and the blow wasn't perpendicular to the car.