Definition
Weight meaning: Weight is the force that pulls an object toward a planet or moon because of gravity. When you stand on a weighing scale and it shows a number, that number tells you how hard gravity is pulling you down toward the ground. The more mass you have, the more gravity pulls on you.
Force of gravity: Gravity is the invisible force that attracts all things toward each other. On Earth, gravity pulls everything toward its center. Weight is how strongly gravity is pulling on a particular object, like your body, a book, or a ball.
Vector quantity: Weight is not just a number—it also includes a direction. That’s why it’s called a vector. Weight always points downward, toward the center of the Earth or the planet you are standing on. It has both size (how strong the force is) and direction (where the force goes).
Difference from mass: Mass is the amount of matter or “stuff” inside an object. Mass never changes no matter where you go. But weight can change depending on where you are because it depends on gravity. For example, your mass is the same on Earth and the Moon, but your weight is less on the Moon because there is less gravity.
Location dependency: Mass always stays the same—it is a measure of how much matter something has. But weight changes depending on the strength of gravity in a location. If gravity is weaker, like on the Moon, you will weigh less. If gravity is stronger, like on Jupiter, you will weigh more.
Inertia vs weight: Inertia is the tendency of an object to resist changes in its motion. It depends on mass, not weight. Weight is the pulling force caused by gravity acting on your mass. So inertia makes you keep moving (or stay still), while weight pulls you downward due to gravity.
Relationship to Gravity
Proportionality: The weight of an object increases if the gravitational field is stronger. That means the heavier the pull of gravity, the heavier the object will feel.
Weight formula: The formula W = mg tells us how to find the weight of an object:
- W stands for weight, measured in newtons (N).
- m is the mass of the object in kilograms (kg).
- g is the gravitational acceleration in metres per second squared (m/s²). So, if an object has a mass of 10 kg and the gravitational acceleration is 9.81 m/s², then the weight is 10 × 9.81 = 98.1 N.
Gravitational acceleration: Gravitational acceleration is how fast an object speeds up when it falls freely due to gravity. On Earth, this value is about 9.81 m/s². It means every second an object falls, its speed increases by 9.81 metres per second.
Earth’s g value: On Earth, g is usually about 9.81 m/s², but this value can change slightly depending on where you are. For example, it is a little different at the mountains, the sea level, or at the poles because of Earth’s shape and rotation.
Pole-equator variation: Gravity is a bit stronger at the North and South Poles than at the equator. This is because the Earth is not a perfect sphere—it is slightly wider around the middle (equator). So, you weigh slightly more at the poles than at the equator.
Other celestial bodies: Different planets and moons have different amounts of gravity. For example, the Moon’s gravity is much weaker than Earth’s, so things weigh less on the Moon. On the Moon, g is only about 1.6 N/kg, which means you weigh about 1/6 of your weight on Earth.
Free fall condition: In a vacuum, which means a place with no air (like space), all objects fall with the same acceleration due to gravity, no matter how heavy they are. A feather and a rock would fall at the same rate if there were no air to slow the feather down.
Mass and weight link: When gravity is the same, the more mass an object has, the more it weighs. For example, a 2 kg object weighs more than a 1 kg object on Earth, because gravity pulls more strongly on the heavier object.
Gravitational field direction: Gravity always pulls objects toward the center of the mass that is creating the gravitational field. On Earth, this means gravity pulls everything down toward the planet’s center.
Field strength definition: Gravitational field strength tells us how much force gravity applies to each kilogram of mass. If the field strength is strong, each kilogram feels a stronger pull.
Field strength formula: You can calculate the strength of gravity in a place by dividing weight by mass: g = W / m. This means that if you know how heavy something is and how much mass it has, you can find out how strong gravity is there.
g equivalence: Gravitational field strength (N/kg) and gravitational acceleration (m/s²) are actually the same in size. So when we say g = 9.81 m/s², we can also say g = 9.81 N/kg.
Units
Weight unit: Weight is a type of force, so its unit is the Newton (N). This unit is named after Sir Isaac Newton, the scientist who discovered gravity and formulated the laws of motion.
Force equivalence: Because weight is a force, it can also be written using its unit as kg·m/s². This shows it depends on both mass (kg) and acceleration (m/s²).
Gravitational field units: The units used to measure the gravitational field strength, g, are N/kg (Newtons per kilogram). But these are the same as m/s², because they both describe how much acceleration or force per kilogram is caused by gravity.
Unit interchangeability: Saying 1 N/kg is the same as saying 1 m/s², because both tell us how much a 1 kg object will accelerate when gravity acts on it.
Key Points to Remember
Definition recap: Weight is the force that gravity uses to pull an object down. It depends on both the mass of the object and the gravity in that location.
Weight equation: The weight of an object is found using the formula W = mg, which means weight equals mass multiplied by gravity.
Earth’s gravity: On Earth, the value of gravity (g) is about 9.81 m/s². We use this number when calculating the weight of things here.
Celestial variation: On other planets or moons, gravity is different. That means your weight will be different too, even though your mass stays exactly the same.
Vector property: Weight always acts downward, in the direction of gravity. That direction is always toward the center of the planet or moon you’re on.
Variable property: Weight changes depending on the strength of gravity. But your mass does not change no matter where you go.
SI unit: The official unit used to measure weight is the Newton (N), and it can also be written as kg·m/s² because it comes from multiplying mass and acceleration.
g unit: The unit for gravitational field strength, which we call g, can be written as N/kg or m/s². Both mean the same thing and tell us how strong gravity is in a place.