explaining on Instagram: "In his famous MIT physics lecture on free fall and weightlessness, Professor Walter Lewin demonstrated the concept in a dramatic and memorable way. He held a stone in his hand and first explained that, while standing still, his hand exerted an upward normal force equal to the stone’s weight (mg), keeping it at rest. Then, to show what happens in free fall, he jumped off a table while holding the stone. During the fall, both he and the stone accelerated downward together