It is an issue of perception within the frame of reference (or place) we are in at the time. Of course, irrespective of these facts, the astronauts on board the ISS (and even the ISS itself) feel ‘weightless’. The gravitational field on the ISS is approximately 89% of that on the Earth’s surface. Both the astronauts and the ISS are in orbit around (or about) the Earth, which also means they are in a continual state of freefall towards the Earth.īut how does gravity itself relate to masses and weights? Equation 5 shows how the weight W of an object can be calculated when you know its mass m and the acceleration due to gravity g. Remember that the astronauts are travelling at the same speed as the ISS. You now know that the ISS orbits the Earth at a distance of 400 km and travels at a speed of 28 000 km/h. It happens whenever an object is in freefall. But what is ‘zero-g’? The gravitational force has not disappeared but there is a feeling of weightlessness. Astronauts frequently train in ‘high g-force centrifuge’ environments to prepare them for space travel. Most people are familiar with the term ‘g-force’ which tells you how many times heavier you feel compared with the everyday experience of 1 g. These forces are balanced, although the speed of the parachutist is high and too dangerous to land! When a parachute is opened, it increases the drag forces to the point where a lower terminal velocity is achieved, and the forces are balanced again (see Week 6). For example, parachutists reach what is known as terminal velocity when the force of gravity is equal to the air resistance. This feeling doesn’t mean that there are no forces, however. ‘Weightlessness’ is where you don’t experience the force of contact, for example when on a fairground ride or skydiving.
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