Weight facts for kids
The weight of an object is how strongly gravity pulls on it. It's a measure of the force that gravity puts on something. It's important not to confuse weight with mass, which is a different idea.
On Earth, the force of weight pulls objects towards the planet's center. For very large objects, like the Moon orbiting Earth, the force pulls towards the center of mass of both objects together.
Many people use the words "mass" and "weight" as if they mean the same thing. They also use units like kilograms or pounds for both. For most of history, weight was measured on Earth. Here, weight is directly linked to mass. If something has twice the mass, it will also have twice the weight. But remember, mass and weight are not the same!
Contents
What Are the Units for Weight?
The official unit for weight in science is the newton. We use the symbol 'N' for it.
In the past, other units were used. For example, the dyne was an old unit of force. Another unit was the kilogram-force. This was the force gravity puts on one kilogram of matter on Earth. A body with a mass of 1 kilogram weighs about 9.81 newtons at sea level.
How Do We Measure Weight?
We usually measure an object's weight using a spring scale. This scale has a spring inside. The spring pushes or pulls against the force of gravity on the object.
Most spring scales show the object's mass, not its actual weight. This is because they are made to be used on Earth. If you took a spring scale to the Moon, it would give you a wrong reading. This is because gravity is weaker on the Moon.
A balance-style weighing scale works differently. It compares the weights of two objects in the same gravitational field. It helps you find out if one object is heavier or lighter than another.
Why Does Weight Change?
Weight is not a fixed property of an object. The local gravitational field changes in different places and at different times. This means an object's weight can change.
- Higher Up, Less Weight: The Earth's gravity gets weaker the further you are from its center. So, an object weighs a little less at the top of a mountain than at sea level. It also weighs less at the equator than at the poles. This is because the Earth bulges slightly at the equator.
- Moon's Pull: Other space objects, like the Moon, also pull on things on Earth. Your weight might be slightly less when the Moon is directly overhead. This is because the Moon's gravity pulls you up a tiny bit.
- Weight on Other Planets: Weight is not just for Earth! An astronaut weighs six times less on the Moon's surface than on Earth. This is because the Moon has much less gravity than Earth.
- Weightlessness in Space: When astronauts or satellites orbit a planet, they seem to be weightless. But they still have weight! Gravity is actually what keeps them in orbit. Objects in orbit move very fast. For satellites orbiting Earth, this speed is about 27,000 kilometers per hour. Without Earth's gravity, they would fly off into space in a straight line. Gravity pulls them towards the planet. The combination of their fast sideways speed and the constant pull towards Earth makes them stay in orbit.
Related pages
Images for kids
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Ancient Greek official bronze weights from around the 6th century BC. You can see them at the Ancient Agora Museum in Athens.
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A weighbridge, which is used for weighing large trucks.
See also
In Spanish: Peso para niños
