Buzz
The Earth's weight is calculated based on the gravitational pull it exerts on nearby objects.
Digital Vision/Getty ImagesA better question might be, "What is Earth's mass?" The simple answer is around 6,000,000,000,000,000,000,000,000 (6 x 1024) kilograms.
An intriguing follow-up question is, "How was this figure determined?" After all, the planet doesn't step onto a scale each day. The weight measurement comes from the gravitational influence Earth exerts on objects near it.
Gravity and Mass
Any two masses in the solar system exert a gravitational pull on each other — this is true for all objects. If you place two bowling balls close together, they will attract each other through gravity. The pull is very weak, but with sensitive instruments, you can measure the gravitational force between them.
By measuring this force, you can calculate the mass of the two objects. The same principle applies to two golf balls, though the force is even weaker because gravitational attraction is proportional to the mass of the objects.
Determining the Mass of Spherical Objects
Isaac Newton demonstrated that for spherical objects, you can simplify the problem by assuming the entire mass is concentrated at the center of the sphere. For Earth, this means treating the mass as if it's all located at the planet's center.
The following equation represents the gravitational force between two spherical objects:
F = G(M1 x M2/R2)
- F is the force of attraction between them.
- G is the gravitational constant equal to 6.67259 x 10 m3/kg.
- M1 and M2 are the two masses that are attracting each other.
- R is the distance separating the two objects.
Calculating Earth's Mass
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 x m/s — we can calculate this force by dropping the 1-kilogram sphere and measuring the acceleration that the Earth's gravitational field applies to it (9.8 m/s).
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).
Mass vs. Weight
Why is it more accurate to discuss the Earth's mass instead of its weight? Weight is a force that requires a gravitational field to be measured. For example, if you weigh a bowling ball on Earth and then on the Moon, its weight on the Moon will be one-sixth of what it is on Earth, but its mass remains unchanged in both places.
To calculate the Earth's weight, we'd need to specify which gravitational field we're using for the calculation. However, the mass of the Earth is a fixed value.
