Wikijunior:Solar System/About gravity, mass, and weight

Please see About weight and gravity for an alternate version of this module.

Do you know your weight in kilograms or pounds? On Earth, your weight is a number. If you are heavier, 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, and so more mass, than the empty box.

Our mass is a number for how much stuff there is in our body. That is, if we grow, adding more cells to our body, or retaining more body fat or increasing how big our muscles are, our mass will increase; the number for your mass becomes larger. If we start a diet, and reduce how much fat is kept by our body, the number for our mass becomes smaller. In countries that use the metric system, also called the "Le Système International d'Unités" or SI, the units of mass used for the weight of people is often kilograms (kg).

Suppose that a child has a mass of 40 kg. Now suppose the child goes to the moon or to a space station, but does not change in body composition. Then the number for how much stuff the body has does not change. The child's mass on the moon or at a space station is 40 kg.

Our mass on any planet on the solar system (Jupiter, Venus, Earth, or anywhere else) is the same. But if we step on a scale, the reading will be different on all of these planets, because a scale measures weight, not mass, and weight depends on gravity as well as on mass.

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 we jump into the air. We cannot fly, but instead we fall and land on the ground. There is a force which pulls us to the ground. This force is called gravity. The Earth makes gravity, so every time that we jump, we will land on Earth again, because the Earth's gravity pulls us, and we are not strong enough to jump fast enough to escape that gravity.

Which objects make gravity? To make significant gravity, an object must have a very large number for mass. A child may only be 40 kg. The Earth's mass is about 6 million billion billion kg. (That's six followed by twenty-four zeros). The Earth has enough mass to make us fall, as quickly as we do, when we jump.

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 pulls on a child, the child also pulls on the Earth. The force on the child is the same as the force on the earth. Because the earth is so massive, that force moves it very little. If we could look very closely, though, we would see that when the child jumps, the Earth is pushed away by the force of the child's legs, a very small distance, mostly it's the child who moves. And then, when the child falls, the Earth also "falls" just a little bit toward the child.

Everything with mass makes gravity. When we are on Earth, the moon, the other planets, and the sun are far away, and the force of gravity gets smaller with distance, so we land on Earth again. (Gravity pulls us towards the center of any massive object. Because we are so close to the center of the Earth, compared to the center of other planets and the Sun, we are mostly pulled toward the center of the Earth when we are on the Earth. Gravity would pull us towards the center of the Venus, or any planet, if we were on the Venus or another planet.)

Suppose we went to the moon. Now the Earth is too far away to have much effect on how we would fall. If we jump from the moon, we will land on the moon again. The mass of the moon is about 70 thousand billion billion kg. This is much less than the mass of the Earth, 81 Moons would have as much mass as the Earth.

The force of gravity varies directly with the two masses that we consider, multiplied together. If we are jumping on the moon, our mass would be the same as on the Earth, but the mass of the Earth is 81 times that of the Moon, so the attractive force, if we were at the same distance from the center of the Earth or Moon, would be 1/81 as great for the Moon as for the Earth. Because the Moon is smaller, though, we are closer to the center, and the gravity on the Moon's surface is about one sixth that of the gravity on the surface of the Earth.

To describe how gravity changes with distance is a little more complicated. If we divide the distance by two, we will increase the force by two times two, or four. That's said this way: "The force of gravity varies with the inverse square of the distance." When the distance is large, a little change in distance makes almost no change in the force of gravity, but the change in gravity from moving from a beach near the ocean to the top of a mountain can be measured. It's not enough to notice just from how it feels to jump!

Force
Gravity exerts a force and that force may be measured in units called "newtons." The force on a mass exerted by gravity will vary with the mass and how strong the gravity is where the mass is located. If we double the mass we will double the force.

However, when we push on an object that is free to move, that object will start to move in the direction we push. How fast it moves depends on three things: how hard we push, the mass of the object, and how long we push.

The more massive the object, the slower will be its response to our pushing. The result is that, although the force of gravity increases with mass, the motion decreases with mass, and these two effects cancel each other out.

So how fast things fall doesn't change with the mass. However, other things may change it, especially friction or air resistance.

A cannonball is pulled more strongly by gravity than a ball-bearing, but it has more mass and takes more effort to start moving. A ball-bearing has less mass, but takes less effort to start moving. Both then take the same amount of time to roll down a ramp, and perhaps you have seen or will see a demonstration of something like this.

Why, then, does something like a feather or piece of paper take so much longer to fall to the ground than a cannonball? This is because the resistance from the air in which they are falling is much greater for the feather or paper than for the cannonball. The resistance is a force opposite in direction to the motion, so it reduces the net force on the object.

(Gravity is always attracting us to the center of the Earth. Even when we are standing on the ground. The ground resists the force of gravity, pushing back up with an equal and opposite force, so we stay put.)

What happens, then, if there is no air? In 1971, astronaut David Scott visited the Moon, where there is no air. He held a feather and hammer, each in one hand, and then dropped them at the same time. They both hit the surface of the moon, also at the same time.

A table showing how gravity changes what happens elsewhere
If we were able to travel to another world, like the astronauts of the Apollo lunar exploration crews, there are a number of things that we would notice that are different from what you would experience on the Earth. There are also some things that would be just the same.

The following is a table regarding what kinds of experiences, if we weigh 40 kg. on Earth, would have if we visited different planets or moons in the Solar System:

In a pressurized chamber like a huge domed city, on the Moon, we would be able to put on wings and flap our arms to fly like birds do here on the Earth. Human powered flight is almost impossible here on the Earth because humans weigh too much here.

Phobos is one of the moons of Mars, and is so tiny that the gravity is very low. For example, if we kicked a ball really hard it could leave Phobos completely and go into orbit as a separate object orbiting Mars. Jumping up would take several minutes before the gravity would pull us back down, so we could jump over a mountain on that moon of Mars if we wanted to.

Of all the objects in the Solar System with a "solid" surface that we could walk on, the Earth has the strongest gravity. Jupiter and Saturn may have stronger gravity, but there is nothing we 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. (However, if there were a floating platform on Jupiter or Saturn, we could walk on it, but it would be difficult, we'd weigh so much.)