Gravity, Mass and Weight

This science project teaches us about the difference between ‘mass’ and ‘weight’.An astronaut has the same amount of ‘stuff’ inside him no matter where he is. He could be on planet Earth…

Neil Armstrong- official portrait in space suit

 He could be walking in space in ‘microgravity’……

astronaut on a space walk with earth underneath him

 He could be walking on the Moon….

astronaut walking on the moon

 He could be walking on Mars…

astronaut walking on Mars

 He could be walking on the surface of the dwarf planet Pluto…

astronaut on pluto with one of its moons charon in the background

He could be trying to walk on the surface of the gas giant Jupiter…..( although this would be impossible since he would quickly sink into a sea of liquid hydrogen)

Jupiter and shadow of one of its moons,Europa

No matter where he is, his mass will always remain constant (stay the same) The amount of ‘stuff’ inside him does not change!

So if his mass always stays the same, what about his weight?

On Earth he weighs 12 stone 1lb (77.1kgs)

Neil Armstrong- official portrait in space suit

 In ‘microgravity’ he would weigh 10 stone 12lbs (69.4 kgs)

astronaut on a space walk with earth underneath him

 On the moon he would weigh 2 stones (12.7 kgs )

astronaut walking on the moon

 On Mars he would weigh 4 stone 7lbs (29 kgs)

astronaut walking on Mars

 On the dwarf planet Pluto he would weigh 0 stone 11lbs (4.9 kgs)

astronaut on pluto with moon Charon in background

 On the ‘surface’ of the gas giant Jupiter he would weigh an amazing 28 stone 9lbs! (182.3 kgs)

Jupiter and shadow of one of its moons, Europa

  • So if an astronaut’s mass remains the same (constant) wherever he is, why does his weight change depending on which planet he stands on?

Let’s see what Homer Simpson thinks!

Homer Simpson’s weight problem

Homer Simpson weighs himself on a set of bathroom scales. He doesn’t look very happy!

Homer Simpson stands on scales looking angry

He weighs 21 stones! (133.8 kgs)

Homer Simpson stands on scales

Homer may believe he is measuring how heavy he is but what he is actually measuring is the force of gravitational attraction between himself and the Earth.

The force of gravitational attraction between Homer Simpson and the Earth just so happens to be called ‘weight’.

We usually measure the force of gravitational attraction in stones or kilograms.

Homer knows that he has eaten too many burgers and hot dogs. He wants to lose weight and lose it FAST!

Homer Simpson looks mad

He thinks carefully about what to do!

Homer Simpson thinking while holding ellbow with finger over mouth

He comes up with a cunning plan!

Homer Simpson looking sly

He knows he could lose weight if he leaves planet Earth…..

Homer Simpson dressed as an astronaut

If he ‘floats’ in orbit 250 miles above the surface of the Earth where the force of gravity is 90% of what it is on the surface of the Earth he would weigh 19 stones (120.4 kilos) He would have lost 2 stones in weight!

Homer in near space

If he walks on the Moon, where the force of gravitational attraction is 20% of what it is on Earth, he would weigh 3 stone 8lbs. (22.6 kilos)

homer simpson walking on the moon

If he walks on Mars, where the force of gravitational attraction is 38% of what it is on Earth, he would weigh 7 stone 12 lbs. (50.4 kilos)

Homer walking on Mars in a spacesuit

If he walks on the dwarf planet Pluto, where the force of gravitational attraction is 8% of what it is on Earth, he would weigh 1 stone 5 lbs. (8.9 kilos)

Homer on Pluto

On the ‘surface’ of the gas giant Jupiter, where the force of gravitational attraction is 254% of what it is on Earth, he would weigh an incredible 49 stone 11 lbs! (316.3 kilos) He would, however, immediately sink into a sea of liquid hydrogen!

Homer sinking into Jupiter

So what is the force that could make Homer Simpson’s weight change depending on which planet he is on?

It’s called G-R-A-V-I-T-Y!

Homer makes a decision

Homer decides that it is not a good idea to lose weight by becoming an astronaut and visiting other planets which have lesser forces of gravitational attraction!

He thinks that the best way to lose weight would be to stay on Earth and stop eating so much!

Homer eating a baguette

Science fair projects

  • Investigate the different forces of gravitational attraction of the planets in our solar system at nssdc.gsfc.nasa.gov. You can measure the different forces of gravitational attraction as ratios, percentages or decimals in comparison with the Earth. Remember that smaller planets can sometimes have more ‘stuff’ packed inside them than larger planets. It’s the mass that is important when calculating the force of gravity, not the size!

Comparison of sizes of planets in our solar system

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