You Asked:

If two bodies fall from the same point, will they reach the Earth at the same time? — Bhargav, Mumbai, India

Marshall Brain Answered:

Let’s say that you climb a flight of stairs and stand on a balcony. In one hand you hold a feather, and in the other you hold a bowling ball. If you drop these two objects at the same time, what will happen? We all know that the bowling ball will reach the Earth first.

The reason the bowling ball arrives first is because the two objects have different terminal velocities. The air in Earth’s atmosphere is a fluid, and the falling objects experience different amounts of drag in that fluid. Because of the fluid, the feather might fall with a terminal velocity of 3 MPH through the air, while the bowling ball reaches a terminal velocity of 150 MPH. The bowling ball hits the ground much sooner than the feather.

This is the basic idea behind a parachute. If two people fall out of an airplane at the same time – one with a parachute and one without – the one with the parachute reaches earth much later than the person without one. The parachute drastically changes the terminal velocity of a falling human.

What if we remove the fluid? If we drop a feather and a bowling ball in a vacuum, they do arrive on the ground at the same time, as famously demonstrated in this video:

Here is the same experiment performed on earth using a vacuum chamber:

A parachute has zero effect in a vacuum because there is no fluid to act on the parachute. The force of gravity is identical on both objects, and therefore they accelerate at the same rate in the gravitational field.