Now let’s get to the bottom line about gravity.

Near Earth, all objects are attracted to the centre of the planet by gravity, or gravitational attraction. Any obstacle an object meets—the ground, a building or a table, for example—prevents it from falling toward the centre. It is the force the obstacle exerts on the object that creates the sensation of weight.
If the object meets no obstacles, it continues indefinitely in free fall toward the centre of the Earth. As it falls, it is in a state of weightlessness.

Surprising though it may seem, gravity and weightlessness are both direct consequences of the Earth’s attraction. The only thing that distinguishes them is the presence of an obstacle.

<sup>Figure 1 - Sitting or standing, these children have the sensation that they are being pulled downward. The floor and the chair are the obstacles preventing them from falling toward the centre of the Earth. This sensation of weight is produced by the force or reaction exerted by the floor or chair on their body. The ball, however, is seen to fall because the walls and floor are not moving.

Figure 2 - Imagine that the ground opens beneath their feet. The whole room would be precipitated toward the Earth’s centre and the children and all the objects in the room would fall at the same velocity (assuming that there is nothing to break their fall). They no longer feel their own weight and therefore have the impression of floating. But they are indeed falling! In other words, they are experiencing weightlessness.</sup>

In the 17^th century, Galileo demonstrated that the velocity of bodies in free fall in a vacuum is independent of their mass. In Earth orbit, astronauts therefore always travel at the same speed, 28,000 kilometres per hour, which explains why a spacecraft and its occupants appear to be motionless relative to one another, and why they appear to be floating.