The scientific definition of weight is the force acting on an object, usually due to gravity, or the force that holds the object in place. However, "weight" is commonly used to mean mass. While they are not exactly the same, since weight can change, on Earth, gravity doesn't vary significantly, so weight and mass are directly proportional. Using weight to mean mass is not technically correct, but it is widely accepted.
Below are a few different types of scales that measure the weight (estimate the mass) of objects sitting on them.
Two common units of mass used to describe the weight of an object include the kilogram (kg), which is the base unit of mass in the International System of Units, and the pound (lb), the unit of mass in the US customary and imperial systems of measurement. They are related to each other as follows:
2.204623 pounds = 1 kilogram
Mass vs weight
Mass (m) measures the amount of matter that makes up an object while weight (W) actually measures how strongly gravity (g) pulls on an object. A key thing about mass is that it does not change. The mass of an object stays constant, regardless where the object is, unless of course you add or remove mass from it. This is not true of weight. Weight changes because the force of gravity can change depending on where the object is; Earth's gravity is weaker the farther away from its center you get, though throughout its surface, the difference is not really significant. The formula for weight is as follows:
W = mg
The abbreviation for gravity, g, is the same as the abbreviation for gram. Be careful not to confuse these two uses.
While gravity throughout the surface of the Earth does not vary much, elsewhere, it can vary much more. For example, the force of gravity on the moon is around that on Earth.
When Apollo 15 was launched from Cape Kennedy, Florida, in July 1971, it carried the first Lunar Rover. This vehicle, which looked a lot like a dune buggy, weighed about 460 pounds on Earth. How much did it weigh on the moon?
The Lunar Rover weighed only 77 lbs on the moon, or 34 kg, compared to 460 lbs or 210 kg on Earth.
Although weight, such as the weight of a person, is often measured in units of mass, such as kilograms and pounds, it is not technically correct. Weight is a force; in the International System of Units, the unit of force is the Newton (N). This can be broken down further into:
where kg is kilograms, m is meters, and s is seconds.
Looking back at the previous formula,
W = mg
this makes sense because kilograms are a unit of mass, which is multiplied by gravity, measured as acceleration, in units of .
When we say that something "weighs" 55 kilograms, that is technically just its mass. We need to multiply it by the force of gravity, which on Earth, is 9.8 Newtons. Someone who weighs 55 kilograms actually weighs:
55 kg × 9.8 = 539 N
Did you know?
You would weigh slightly less at the top of Mount Everest, Earth's highest mountain above sea level, than you would at sea level because the gravity at the summit is a little less than 1 g. This is because the force of gravity exerted on an object gets smaller the farther the object gets from the center of the Earth.