The Acceleration of Freefall
In 1589, a scientist called Galileo Galilei was reported to have climbed up the leaning tower of Pisa and then dropped a small object and a heavier object at the same time to show that they both fell at the same rate.
This is true - all objects fall at the same rate! However, a lot of people didn't believe him and it's easy to understand why.
If you drop a hammer and feather at the same time, then of course the hammer will accelerate faster and hit the ground before the feather. What people didn't realise is that the upwards effect of air resistance on the feather is much greater than on the hammer. It effectively balances the feather's small weight and slows the feather much more than the hammer (which has a much larger weight).
This was beautifully demonstrated in the Apollo 15 Moon landing when one of the astonauts demonstrated the hammer and feather experiment (see the video below).
The reason this experiment worked so well on the Moon is that there is no air on the Moon, and so no air resistance. The feather and the hammer fell at the same rate and hit the ground at the same time.
The acceleration of a body in freefall near the Earth is constant. Freefall is the term given to any object that is freely falling towards the Earth (or any other planet).
This means that each second, the downwards speed of the falling body increases by the same amount. This is called a constant acceleration and occurs as long as air resistance on the body can be ignored.
In the case of a feather, air resistance cannot be ignored. However, for a golf ball, or a canon ball (at low speeds), the effect of air resistance is very small compared to the weight of these objects.
This is a key physics principle, discovered by Galileo: Every object falls at the same rate, as long as we can ignore air resistance.