Do
you know your weight in kilograms or pounds? On Earth, your weight is a number.
If you are more heavy, then this number is larger. If you go to the moon, or to
a space station, is this number the same? Some people say that it is the same,
and some people say that it is different. To understand why, you need to know
about gravity, mass, and weight. Mass Like
many words, the word weight can have several different meanings. One meaning
of weight is called mass. The word "mass" is used in astronomy and other
parts of science. The
mass of an object is simply the amount of material the object is made of. The
more material the object is made of, the more mass it has. Things that have a
big mass are harder to move and harder to stop than objects with just a little
bit of mass. So an empty box (with only air inside) is easier to move than a box
filled with books. The box with books has more material, more mass than the empty
box. Your mass
is a number for how much stuff your body has. That is, if you eat much food, your
mass will increase; the number for your mass becomes larger. If you start a diet,
your mass will decrease; the number for your mass becomes smaller. In countries
that use the metric system, also called the "SystÃ¨me Internationale" or SI, the
units of mass are often in kilograms (kg). Suppose
that one child had a mass of 40 kg. Remember, the word "weight" can mean "mass",
so the weight of the child is 40 kg. Now
suppose that you go to the moon or to a space station, but you do not eat too
much or start a diet. Then the number for how much stuff your body has, this number
does not change. The child's mass on the moon or at a space station is 40 kg.
The child's weight at these places is 40 kg. Your
mass on any planet on the solar system (Jupiter, Venus, Earth, or anywhere else)
is the same. Your weight is the same on all of these planets. More
about Mass -- an interactive activity GravityThere
is another meaning of "weight", called "force of gravity". The word "weight" can
mean one of two things, "mass" or "force of gravity". But what is "gravity"? Suppose
you jump into the air. You cannot fly, but instead you fall and land on the ground.
There is a force which pulls you to the ground. This force is called gravity.
The Earth makes gravity, so every time that you jump, you will land on Earth again,
because the Earth's gravity pulls you. Which
objects make gravity? To make gravity, an object must have a very large number
for mass. A child is only 40 kg. Earth is about 5974200000000000000000000 kg.
The Earth has enough mass to make children fall when they jump. Actually,
all things with mass make gravity and attract one another. The more mass an object
has, the more it attracts other objects toward it. So while the Earth can pull
on a child, the child can also pull on the Earth. Because the child is only 40
kg, the child makes almost no gravity, and no person sees the Earth move. The
moon, the other planets, and the sun each have enough mass to make gravity. When
you were on Earth, the moon, the other planets, and the sun were too far away,
so you landed on Earth again. (Gravity pulls us towards the center of the Earth
when we are on Earth. It would pull us towards the center of Venus if we were
on Venus.) Suppose
you went to the moon. Now the Earth is too far away. When you jump from the moon,
you will land on the moon again. The mass of the moon is about 73600000000000000000000
kg. This is less than the mass of the Earth. The consequence is that the moon
makes less gravity than the Earth. The force of gravity on you at the moon is
less than the force of gravity on you at the Earth. NewtonsThe
force of gravity on you is also a number. In the SI system, we can measure this
force in newtons (N). Some people have tried to measure mass in newtons
or to measure force in kilograms, but that does not work well. Remember that weight
can either mean mass or force. So whenever we measure weight in kilograms, we
are actually measuring mass, but whenever we measure weight in newtons, we are
measuring the force of gravity that pulls something. (Sir
Isaac Newton lived in England in the 1600s and 1700s. The name of the "newton"
is after him. Maybe you have heard the story about the apple falling on his head,
helping him to learn how gravity works.) Sometimes
the word "weight" is used for the mass that we measure in grams. Sometimes the
word "weight" is used for the force we measure in newtons. A
table showing how gravity changes what happens elsewhereIf
you are able to travel to another world, like the astronauts of the Apollo lunar
exploration crews, there are a number of things that you would notice that are
different from what you would experience on the Earth. There are also some things
that would be just the same, so it would take a little bit of getting used to
all of those changes. The
following is a table regarding what kinds of experiences you would have if you
visited different planets or moons in the Solar System:
| Earth | Moon | Mercury | Venus | Mars | Pluto | Phobos |
Surface Gravity (compared to Earth) | 1.0 | 0.17 | 0.38 | 0.90 | 0.38 | 0.06 | 0.005
(average) | Your weight (mass) | 40 kg | 40
kg | 40 kg | 40 kg | 40 kg | 40 kg | 40 kg |
Energy needed to stop 1 kg ball moving at 90 km/hr | 625
joules | 625 joules | 625 joules | 625 joules | 625 joules | 625
joules | 625 joules | How much you can
lift | 10 kg | 60 kg | 30 kg | 10 kg | 30 kg | 170
kg | 19 000 kg | How high you can jump | 20
cm | 120 cm | 53 cm | 22 cm | 53 cm | 340 cm | 400
m | How long it takes to fall back to the ground | 0.4
s | 2.4 s | 1.1 s | 0.4 s | 1.1 s | 6.8 s | 6.4
minutes | How far you can kick a ball | 20
m | 120 m | 53 m | 22m | 53 m | 340 m | 38
km (into Martian orbit) | Note that on
the gas giants such as Jupiter, Saturn, Uranus and Neptune you probably wouldn't
be able to find a place to sit down or do these things. | One
very interesting thing about living on the Moon is that in a pressurized chamber
like a huge domed city, you would be able to put on wings and flap your arms to
fly like birds do here on the Earth. Human powered flight is almost impossible
here on the Earth because you can't generate the energy necessary to fight gravity
pulling you down.
Phobos
is one of the moons of Mars, and is so tiny that the gravity almost doesn't even
exist. For example, if you kick a ball real hard it will leave Phobos completely
and instead go into orbit as a seperate object orbiting Mars. Jumping up would
take several minutes before the gravity would pull you back down, so you could
jump over a mountain on that moon of Mars if you wanted to. It would also be possible
to jump too high and leave that moon altogether. It
is also interesting to see that of all the objects in the Solar System with a
"solid" surface that you can walk on, the Earth has the strongest gravity. Jupiter
and Saturn may have stronger gravity, but there is nothing you can say is a "solid"
surface to walk on. There may be planets that are larger than the Earth with a
solid surface, but they are not found in our Solar System. A
table with force in newtonsLet's
say that, on the Earth, your bathroom scale says you weigh 86 pounds (lb), which
is just over 39 kilograms (kg).
Weight on the Moon or other planets Earth | Moon | Mercury | Venus | Mars | Pluto |
86 lb | 86 lb | 86 lb | 86 lb | 86 lb | 86
lb | 39 kg | 39 kg | 39 kg | 39 kg | 39
kg | 39 kg | 6 st 2 lb | 6 st 2 lb | 6 st 2
lb | 6 st 2 lb | 6 st 2 lb | 6 st 2 lb | 382.2
N | 62.4 N | 140.4 N | 339.3 N | 144.3 N | ? |
Note that on the gas giants such as Jupiter, Saturn, Uranus,
and Neptune you won't be able to find a place to set down your scale and weigh
yourself. | On
Earth, if you weigh 51 kilograms or 112 pounds, your weight is 500 newtons. We
use this "weight" when we are really interested in how hard we push down on something
because gravity is pulling us.
Your
weight in Newtons would be a lot less on the Moon, about one-sixth of what it
is on Earth. To find your weight in Newtons as you stand on the Moon, just take
your mass on Earth, and if that mass is in kilograms, multiply it by 1.6. That
will give you your weight on the moon in Newtons. But then you'll have to add
in the weight of your spacesuit as well, because that is pressing down on the
surface just like you are. When
scientists want very precise weights, they have to take into account more forces
than only gravity, like atmosphere, rotation, and free fall. Another
table with force in pounds-force and newtons
Weight on the Moon or other planets | Earth | Moon | Mercury | Venus | Mars | Pluto |
Your real weight | 86 pounds | 86 pounds | 86
pounds | 86 pounds | 86 pounds | 86 pounds | 39
kilograms | 39 kilograms | 39 kilograms | 39 kilograms | 39
kilograms | 39 kilograms | 6 stone 2 lb | 6 stone
2 lb | 6 stone 2 lb | 6 stone 2 lb | 6 stone 2 lb | 6
stone 2 lb | Your weight in astronomer
talk | 86 pounds-force | 14 pounds-force | 32 pounds-force | 78
pounds-force | 32 pounds-force | 5 pounds-force |
380 newtons | 63 newtons | 144 newtons | 350 newtons | 144
newtons | 23 newtons | Note that on the gas giants
such as Jupiter, Saturn, Uranus, and Neptune you won't be able to find a place
to set down your scale and weigh yourself. | SEE
ALSO: MASS MASS
VS. WEIGHT |