Explaining Everything on Instagram: "In one of his most memorable MIT lectures, Walter Lewin demonstrated the concept of free fall and apparent weightlessness in a dramatic way. While standing still, Lewin held a stone in his hand and explained that the stone remained at rest because his hand provided an upward normal force equal to the stone’s weight (mg). To show what changes in free fall, he then jumped off a table while still holding the stone. During the fall, both Lewin and the stone accel