Weight

Weight is a basic idea that helps us understand how things behave because of gravity. In science and engineering, weight is the force that gravity pulls on an object. This force changes depending on where the object is and how strong gravity is there.

Some books say weight is a force that pushes in a certain direction, while others say it is just how strong that force is. When people measure weight with a scale, they are measuring how much force the scale needs to push back against gravity. If an object is falling freely, like an apple dropping from a tree, it feels weightless for a moment because there is nothing pushing back against gravity.

We measure weight in newtons. For example, an object that has a mass of one kilogram weighs about 9.8 newtons on Earth but only about 1.6 newtons on the Moon because the Moon’s gravity is weaker. Even though scientists keep weight and mass as different ideas, many people use the words interchangeably in daily life. Understanding weight can get more complex when we think about how space curves because of gravity, an idea from the theory of relativity.

History

People have talked about how heavy or light things are for a very long time. Early thinkers like Plato and Aristotle believed weight was a natural part of objects. Archimedes studied how weight and floating worked together.

Later, scientists like Galileo learned more about weight. Then came Newton, who helped us understand that weight is different from mass. He showed that weight depends on gravity pulling on an object.

In the 20th century, new ideas about space and time changed how we think about weight. Even though it can be tricky, weight is still an important idea in learning about physics.

Definitions

There are a few ways to think about what weight means.

The most common way is to think of weight as the pull of gravity on an object. We can see this in the formula W = mg, where W stands for weight, m for the mass of the object, and g for gravitational acceleration. In 1901, experts agreed that weight is a force, connecting it to mass and gravity.

Another way to think about weight is by measuring how much a thing pushes down on a scale. This number can change depending on where you are — for example, an object will weigh less on the Moon than on Earth. But this way of measuring does not change the actual pull of gravity on the object.

Mass

Main article: Mass versus weight

In science, weight and mass are different. Mass is a property of matter that stays the same no matter where you are. Weight is the force of gravity pulling on that matter.

People often say "weight" when they mean "mass." For example, someone might say an object "weighs one kilogram," but a kilogram is actually a unit of mass.

Because gravity is almost the same everywhere on Earth, the difference between mass and weight doesn’t matter much for everyday use. If one object weighs ten times as much as another, it also has ten times the mass. This is why using a scale to "weigh" something is a good way to measure its mass.

In science, weight is measured in newtons (N), which are a unit of force. But in everyday life, people usually talk about weight in kilograms (kg), which are a unit of mass. Different places can use other units too.

Sensation

See also: Apparent weight

The feeling of weight comes from a push of fluids in the inner ear. This area is called the vestibular system. It has tubes that help us sense movement. What we feel as weight is the effect of forces on our body. This can happen when we stand still because of gravity or when we move quickly in a car or elevator.

Measuring

Main article: Weighing scale

"Weigh" redirects here. For other uses, see Weigh (disambiguation).

We measure weight in two main ways. One way uses a special scale that shows how strong gravity is pulling on an object. These scales might show slightly different numbers in different places because gravity changes a little.

Another way to measure weight uses a balance. A balance compares the weight of something unknown with known objects. Balances give the same answer no matter where you are because gravity pulls on both the same way. Balances are often marked with units of mass. If you want to know the force of gravity on an object, you can figure it out using its mass.

In shopping, we talk about gross weight, which is the total weight of a product including its packaging. Net weight is the weight of the product itself, without the packaging. Tare weight is the weight of the packaging alone.

Relative weights on the Earth and other celestial bodies

Main articles: Earth's gravity and Surface gravity

The table below shows how strong gravity feels on the Sun, the Moon, and each of the planets in our Solar System. For big planets like Jupiter and Saturn, we look at the top clouds. For the Sun, we look at its outer layer called the photosphere. The numbers are close to what you would feel near the poles of these places.