Weight

Weight is a basic idea that helps us understand how things behave because of gravity. In science and engineering, weight refers to the force that gravity pulls on an object. This force depends 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 really 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 also gets more complex when we think about how space curves because of gravity, a idea from the theory of relativity. Teachers have debated for many years about the best way to explain weight to students, and today several different ideas about weight are used in different situations.

History

People have talked about how heavy or light things are since ancient times. Early thinkers like Plato and Aristotle thought weight was a natural part of objects. Archimedes looked at how weight and floating worked together.

Later, scientists like Galileo studied weight more closely. 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’s a bit tricky now, weight is still an important idea in learning about physics.

Definitions

Several ways exist to describe what weight means, and they are not all the same.

The most common way to think about weight is 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, a group of experts agreed that weight is a force, linking it directly to mass and gravity.

Another way to think about weight is by measuring how much a thing pushes down on a scale. This measurement can change depending on where you are — for example, an object will weigh less on the Moon than on Earth. However, this way of measuring does not change the actual gravitational pull 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. In everyday talk, people often use "weight" when they really 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. The Earth’s gravity isn’t exactly the same everywhere, but for most purposes, it doesn’t make a big difference.

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 the push of fluids in the inner ear, in a special area called the vestibular system. This system has tubes that help us sense movement. What we feel as weight is actually the effect of forces on our body, whether we are standing still because of gravity or moving 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 spring scale or a special type of scale that measures how strong gravity is pulling on an object. These scales can show slightly different numbers depending on where you are because gravity can change a little from place to place. To make sure the scales show the right numbers, they are adjusted at the factory. But if you move the scale to a new place, it might need to be adjusted again to be very exact.

Another way to measure weight uses a balance. A balance compares the weight of something unknown with the weight of known objects. Because gravity pulls on both the unknown object and the known objects the same way, the balance gives the same answer no matter where you are on Earth. Balances are often marked with units of mass, which tell you how much stuff is in the object, instead of its weight. If you need to know the real force of gravity on an object, you can figure it out by multiplying its mass by how fast gravity pulls it down.

In shopping and trade, we often talk about gross weight, which is the total weight of a product including its packaging. Net weight is just 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 compares how strong gravity feels on the Sun, the Moon, and each of the planets in our Solar System. For the big planets like Jupiter and Saturn, we look at the top clouds instead of their solid surfaces. For the Sun, we look at its shining outer layer called the photosphere. The numbers shown are close to what you would really feel near the poles of these places.