A very small girl proudly jumps from the sidewalk onto the pavement. Again and again she lets herself fall from a height that is almost half her own. In a few years’ time, she will be using skipping ropes, slides and swings, and will learn to enjoy that strange, fleeting sensation that lasts less than a second: our hair stands on end, our arms and legs feel light, our stomach rises, out of place. Perhaps, as a teenager in a theme park, she will dare to plunge from a 100-metre tower, satisfying this desire that others satisfy through sport (ski jumping, trampoline jumping, bungee jumping), emulating – albeit on a small scale – Felix Baumgartner, who in 2012 jumped from a balloon 39 km high. But the time record is held by the crew of the International Space Station (ISS): Samantha Cristoforetti, commander of the Minerva mission, has spent a total of 370 days, 5 hours and 45 minutes in space.
I drop something from a certain height, for example a stone. Suppose there is no friction with the air. The Earth’s gravity accelerates it: it falls with increasing speed (g is about 10 m/s2) and hits the ground just below where I dropped it. If I throw it forward with my hand, as it falls and accelerates, it will also move horizontally at a constant speed. Like any thrown object or projectile, it follows a parabolic line and hits the ground at a certain distance. If I use a slingshot, like in the famous video game Angry Birds, it will hit even further. The fall time is always the same, but the horizontal speed is higher and it travels a longer distance in that time. If we increase the speed more and more, we move the point of impact further and further away until… the stone falls, but it never hits the ground because the Earth is spherical and its surface is curved: and so the stone orbits. Galileo Galilei had already challenged the Aristotelian division between a perfect and unchanging supralunar world of heavenly bodies and a sublunar, terrestrial and corruptible one – which he liked much better. Newton went further: we can think of a stone, or we can think of the Moon, which also falls or orbits the Earth. Once this conceptual barrier has been overcome, we can think of the Earth in free fall around the Sun, the Sun around the centre of our galaxy, and so on.
Now imagine that the projectile is a box and you are inside it, standing on the floor. We drop it, like a lift whose cable is cut (do not worry, this is just a thought experiment) or we throw it like a projectile. What do you see and feel? Your hair stands on end, your limbs become lighter, your stomach rises and you seem to float, you do not feel any weight because the box is accelerating downwards just like you, at about 10 m/s2. The reason is that our perception of weight actually comes from the forces that support us: the tension of your arms on your joints, the tension of the seat on your buttocks or the contact of the ground on your feet. This is the force that a scale measures when it reads 60 kg in your bathroom – and zero in the box in free fall, in the «absence of weight». This box exists in Bordeaux and it is called the Airbus A300 Zero-G. After taking off and rising to a height of about 8 km over the Bay of Biscay, it performs twenty-second drops followed by climbs, in several repeated series. It makes it possible to carry out «microgravity» experiments which, due to their size or duration, are not suitable for free-fall towers such as the Fallturm in Bremen (Germany). It also allows the crew of future space missions to train in the sensation they will experience continuously in orbit inside the ISS, another box at an altitude of 400 km, where the acceleration of gravity is only slightly less than at sea level, about 8.7 m/s2.
Weightlessness: lightness as opposed to heaviness. In free fall, when gravity is the only force at work (almost all dictionaries define it incorrectly). The semantic drift in science is curious. And these are also subtle worlds, like soap bubbles.
With a mobile camera or a GoPro, you can record a video with a frame rate N > 25 fps. When you play it back, you will see the movement slowed down in the same proportion (what takes 1 second will slow down when you play it back at 125/25 = 6 seconds, for example, if N = 125 fps):
- Record the fall of an object (a marble or a stone) by placing a tape measure next to it to measure the distance it travels. For example, after 0.1 s (0.6 in the video) the stone travels 5 cm and after 0.3 s (1.8 in the video) it travels about 45 cm. The stone accelerates (its speed increases as it falls). Using the latter value, we obtain, for example, the acceleration due to gravity a = 2d/t2 = 2-0.45/0.32 = 10 m/s2.
- Put an object in a transparent plastic box. Drop the can from a certain height and record the fall on video. When you play back the video, what happens to the object if you throw it upwards or horizontally?