This science project improves our understanding of the force of gravity. More specifically, we investigate ‘weightlessness’ and why there appears to be little or no gravity in space at a distance of just 250 miles (402 kms) above the surface of the earth.
This article follows on from Understanding Gravity and Mass in which we explain the meaning of ‘mass’.
Gravity revisited
In order for a force of gravity to be present there must be a ‘force of attraction’ and some physical ‘movement’ between two objects. To notice any force of gravity an object with a small mass must be pulled towards the center of another object with a far larger mass.
In the image below ‘base’ jumpers are being pulled towards the center of the earth. The center of the earth is located some 3959 miles (6371 km) down below!
We know how the force of gravity affects us all, living as we do, on the surface of planet earth.
But do we know how the force of gravity affects us if we are 250 miles above our planet? We probably know very little since not many of us get the chance to go there!
- How can there be any force of gravity in space if astronauts are able to ‘float’ 250 miles above the surface of the earth?
- How can there be any force of gravity at this height (altitude) if the International Space Station is able to ‘float’ in a low earth orbit?
- On the other hand how can there NOT be any force of gravity given that the moon orbits the earth at distance of 250 000 miles? (402 336 kms) If there were no force of gravity, the moon would not be orbiting the earth!
- Before we answer this question we need to understand how the force of gravity makes it difficult for us humans to reach space.
Force of gravity-reaching space
To reach space a rocket has to escape the force of earth’s gravity. Before it can escape the force of earth’s gravity a rocket has to accelerate to very fast speeds.
To attain these incredibly fast speeds a spacecraft has to carry enormous external storage tanks to provide rocket fuel to power very thirsty engines.
Once empty, these external fuel tanks are no longer of use and are jettisoned into the sea. (pulled down by the force of gravity!)
By the time it reaches a height (or altitude) greater than 62 miles (100kms) above the surface of the earth the space shuttle will be able to switch off its engines and ‘float’ in space. However, it will only be able to ‘float’ in space so long as it is orbiting the earth at a fast enough velocity. (more of which later)
The space shuttle does not now face any immediate risk of being pulled back down to earth by the force of gravity.
Reaching orbit from the surface of the moon, the sun, Jupiter and Mars
The force of gravity is 83% weaker on the moon than it is on planet earth. If a space craft wishes to escape the force of the moon’s gravity so it can orbit the moon without its engines on it would need to accelerate to 8,640 mph. (13905 kmh)
The force of gravity on the surface of Jupiter is 2.5 times greater than the force of gravity on the surface of planet earth. A rocket blasting off from the surface of Jupiter (if that were ever possible!) would have to accelerate to an ‘escape speed’ of 133,097 mph (214, 200 kmh) so it could reach an altitude at which it could orbit Jupiter with its engines switched off.
The force of gravity on the surface of the sun is 28 times greater than on planet earth. Our pretend rocket blasting off from the surface of the sun would have to reach an incredible escape speed of 1,381,308 mph (2,223,000 kmh) before it could reach an altitude at which it could orbit the sun with its engines switched off.