Two balls of identical size and shape are dropped from the same height. One ball has a mass of 50kg and the other has a mass of only 10kg. Why do they reach the ground at the same time?

The only forces acting on the balls are gravity (pulling down) and air resistance (pushing up). Since the balls are the same shape and size, the air resistance will be the same on both. Likewise, the gravitational force is constant on Earth. Therefore the two balls experience exactly the same forces, so they fall at the same rate and hit the ground at the same time.