Two falling balls of different masses
Q: In DR we dropped 2 balls of different masses from the same height and saw that they hit the floor at the same time. This is confusing to me - shouldn't the lighter ball hit first since it has less inertia and the same gravity as the heavier ball?
A: I believe the confusion here is due to a loose use of the word gravity. When we say gravity we may either mean the acceleration due to gravity, which refers to the g=9.8 m/s^2 acceleration experienced by all objects on earth, or we may be referring to the force due to gravity, by which we mean F=mg=weight. When we drop the two balls they both experience the same acceleration due to gravity, 9.8m/s^2, which is why they hit the ground at the same time. However, the force of gravity is different for each ball since F=mg and the masses are different. The less massive ball has a smaller inertia and is easier to accelerate, but it also has a smaller force due to gravity on it. The result is that both balls have the same acceleration due to gravity and thus hit the floor at the same time.