About Gravity, Mass and Weight

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 "Système 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

Gravity

There 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.

Newtons

The 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 elsewhere

If 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:

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 newtons

Let's say that, on the Earth, your bathroom scale says you weigh 86 pounds (lb), which is just over 39 kilograms (kg).

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

SEE ALSO:

MASS

MASS VS. WEIGHT