# How much does planet Earth weigh?

It would be more proper to ask, "What is the mass of planet Earth?"1 The quick answer to that is approximately 6,000,000, 000,000,000,000,000,000 (6 x 1024) kilograms.

The interesting sub-question is, "How did anyone figure that out?" It's not like the planet steps onto the scale each morning before it takes a shower. The measurement of the planet's weight is derived from the gravitational attraction that the Earth has for objects near it.

It turns out that any two masses have a gravitational attraction for one another. If you put two bowling balls near each other, they will attract one another gravitationally. The attraction is extremely slight, but if your instruments are sensitive enough you can measure the gravitational attraction that two bowling balls have on one another. From that measurement, you could determine the mass of the two objects. The same is true for two golf balls, but the attraction is even slighter because the amount of gravitational force depends on mass of the objects.

Newton showed that, for spherical objects, you can make the simplifying assumption that all of the object's mass is concentrated at the center of the sphere. The following equation expresses the gravitational attraction that two spherical objects have on one another:

F = G(M1*M2/R2)

• F is the force of attraction between them.
• G is a constant that is 6.67259 x 10-11 m3/kg s2.
• M1 and M2 are the two masses that are attracting each other.
• R is the distance separating the two objects.

Assume that Earth is one of the masses (M1) and a 1-kg sphere is the other (M2). The force between them is 9.8 kg*m/s2 -- we can calculate this force by dropping the 1-kg sphere and measuring the acceleration that the Earth's gravitational field applies to it (9.8 m/s2).

The radius of the Earth is 6,400,000 meters (6,999,125 yards). If you plug all of these values in and solve for M1, you find that the mass of the Earth is 6,000,000,000, 000,000,000,000,000 kilograms (6 x 1024 kilograms, or 1.3 x 1025 pounds).

1It is "more proper" to ask about mass rather than weight because weight is a force that requires a gravitational field to determine. You can take a bowling ball and weigh it on the Earth and on the moon. The weight on the moon will be one-sixth that on the Earth, but the amount of mass is the same in both places. To weigh the Earth, we would need to know in which object's gravitational field we want to calculate the weight. The mass of the Earth, on the other hand, is a constant.

Originally Published: Jun 30, 2009

### Mass of Earth FAQs

##### How is Earth mass calculated?
Since scientists already know the radius of planet Earth, they used the Law of Universal Gravitation to determine Earth's mass with respect to the gravitational force on an object on the Earth's surface. Simply put, this method uses Earth's radius as the distance.
##### What is most of Earth's mass?
Earth's core is 15 percent of the planet's volume but 30 percent of its mass. Most of Earth's mass is its mantle. The mantle is 84 percent of its volume and nearly 70 percent of its mass. Earth's crust is just 1 percent of its mass.
##### How did Cavendish calculate the mass of the Earth?
Cavendish measured the force between attracting metal spheres with the help of a torsion balance. He already knew the mass of both spheres and the distance between them. By carefully measuring the force between these spheres, he calculated their gravitational attraction. Since the gravitational force of Earth on the smaller ball could be measured by weighing it, the ratio of the two forces allowed the density of the earth to be calculated, using Newton's law of gravitation.
##### What is the simple definition of mass?
Mass is the measure of the amount of matter in any object. On Earth we can weigh things to determine mass. But in outer space, something might have a lot of mass but not much weight because the pull of gravity would be small.