This demonstration shows how gravity affects two balls of equal size and different mass. Ask your students which ball will hit the ground first, the heavy one or the light one. Drop the balls and have your students observe the results. Some students will disagree so it may be necessary to repeat the experiment several times. You should observe that the balls hit the ground at the same time.
Gravity causes objects to accelerate towards the ground.
As a result, for each second both balls fall, they gain the same amount of speed. If the speed of both balls is equal, they will travel the same distance for a given amount of time. Since they are falling the same distance, it will take the same amount of time for each to hit the ground. This experiment can be expanded by using a basketball and a beach ball. Repeat the expeiment and see which one hits the ground first.