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on Different Planets
Comparison of different planets' gravity on the surface, some with more gravity than the Earth and some with less gravity than the Earth:
One of the first things you notice when you arrive on a planet is the gravity. It seems you weigh more or less depending on how much gravity there is. On some planets you can leap about because the gravity is weak, whereas on some you need to find a comfy chair quite soon because the gravity is so strong.
Of course you should read the travel brochure before going, and give it a try in the simulator, so you have a good idea what to expect.
Here's a helpful table showing the Gravity of different Planets:
(relative that of Earth)
These figures have been adjusted with Earth=1 so as to give a comparison, for example Lunar gravity is one sixth of that of Earth, etc.
The gravity on the surface of a planet is dependent on various things: How heavy the planet is (the heavier the planet, the more mass it has, the more gravity it has). Also, how small it is. A small dense planet can have a surprisingly high gravity at the surface. In comparison, a planet of the same mass as the Earth, but made of expanded polystyrene, would be much bigger and have a much lower gravity on the surface. Also see list of densities
You can work out the gravity of a planet by using the formula g = G * M / R2 where G is the Universal Gravitational Constant 6.67 x 10-11 and M is the mass of the planet in Kg and R is the radius of the planet in metres. If you do this for the Earth you get 6.67 x 10-11 x 5.972 x 1024 / ( 6378 x 103 )2 = 9.79 (not far off the 9.81 you see in physics books). The equation produces results measured in Newtons per Kilogramme, so if you want to get figures relative to the Earth gravity like in the table, you can divide the result by 9.81. Try this for the Moon and you get 6.67 x 10-11 x 7.35 x 1022 / ( 1738 x 103 )2 = 1.62 , divided by 9.81 = 0.165 = about one-sixth of the gravity of the Earth as expected. You can see how useful scientific calculators and exponential notation are!
Gravity figures for planets, such as 9.81 for the Earth, are measured in metres per second per second. 9.81 m/s/s or 9.81 ms-2 is the rate at which a falling object would accelerate. Or to put it another way, a falling object would be travelling 9.81 metres per second faster for every second it continued to fall. In practice, falling objects on the Earth tend to reach their terminal-velocity because of air friction. The figures in the chart have been adjusted by dividing them by 9.81 to give figures which compare with the Earth's gravity =1 which folks are familiar with.
Other equations which are some help in working out things to do with planets are: The volume of a sphere V = 4/3 x PI x R3 (planets are assumed to be spherical), and density = mass / volume .
Yes, you're welcome to use this page for your educational project, but please include the fact that it's available at www.zyra.org.uk/gravities.htm
* Planet : The term used loosely, as there are plenty of places you can land on which have some approximation to the idea that you're on a planet. Generally if they are round and have a surface (or somewhere a surface might be), then they get included in lists like this where the idea of "Gravity on the Surface" has some meaning. Some people have even funnier ideas about "what makes something a planet?" - see the Pluto Problem
*1 : Planets (and other things) which have no actual solid surface to land on. Hotels in Airships floating about in the atmosphere still have carpets with apparent "gravity on the surface". You can call them submarines or ships if you like, there's still an apparent gravity as if you're "ON" a planet. For convenience, the "surface" is often assumed to be somewhere at about the height (altitude) where the atmospheric pressure is similar to that of the Earth.
*2 : If you live on a planet where the gravity is unreasonably high (such as the Earth) then it may seem a bit "depressing". Solutions to relieve the problem temporarily include getting a trampoline (where the energy gained in falling is converted back into kinetic energy upwards), acquiring a medium of appropriate density such as that in items at Pool Center, and going in for activities such as skydiving (see activity sports).
Other gravity related explanations include How things stay up in orbit
I have heard that there is a brilliant hands-on science-museum style demonstration of gravity on different planets, where cans of baked beans are on show with simulated gravities as per different planets. You can lift them and feel what the gravity is like, and feel for yourself a gravity comparison using an everyday object, a can of baked beans!
How High Can You Jump on Other Planets?
To work this out, you first need to know how high you can jump on Earth, but you have to calibrate this by a technique which is different from that used in the traditional sport of High Jump. The question is not "How high a bar can you jump over?" but "How high would a ceiling have to be so that you could jump and not quite hit your head on the ceiling?". To test this without hurting your head on any low ceilings and without leaving any dints in polystyrene ceiling tiles, an adequate measure can be found by using a piece of string between two measuring poles. Find the maximum height you can nut, and then subtract your own height. For example, if you're 5ft 6in tall and you can jump so your head reaches a height of 6ft 6in, then you can jump 1ft. Then, to work out how far you could jump on a different planet with different gravity, you divide that by the gravity of the planet. On a planet that's got half the gravity of the Earth, you could jump twice as high, ie 2ft.
If the Gravity is very weak, can you jump off the Planet?
Tourists visiting all-inclusive resorts on Ceres, for example, might be concerned that a gravity of less than one twentieth that of the Earth, could mean that there's a danger of jumping "off" the planet?! However, to jump off a planet (in one go) requires achieving the Escape Velocity. Even for a low gravity minor planet such as Ceres the escape velocity is hundreds of MPH. Escape velocity is a function of the gravity well, not the force of gravity at the surface. It's more difficult to escape from Saturn than from Earth, because in contrast to the g-force at the "surface", the gravitational well of Saturn is much deeper.