Weight
Weight is the gravitational force exerted on an object due to a gravitational field, typically Earth's. It is a vector quantity directed toward the centre of the gravitational body and varies depending on the local gravitational acceleration. A person who weighs 686 N on Earth would weigh only about 114 N on the Moon, because the Moon's gravitational acceleration is approximately one-sixth that of Earth.
Key Formula
W = m × g
LaTeX: W = mg
| Symbol | Meaning | Unit |
|---|---|---|
| W | Weight (gravitational force) | Newton (N) |
| m | Mass of the object | kilogram (kg) |
| g | Acceleration due to gravity | m/s² (9.8 m/s² on Earth's surface) |
Worked Example
Problem
A student has a mass of 55 kg. What is the student's weight on Earth where g = 9.8 m/s²?
Solution
Using W = m × g: W = 55 kg × 9.8 m/s² = 539 N.
Answer
The student's weight on Earth is 539 N (approximately 539 newtons downward).
Weight of a 70 kg Person on Different Celestial Bodies
| Location | g (m/s²) | Weight W = mg (N) | Compared to Earth |
|---|---|---|---|
| Earth | 9.8 | 686 | 100% |
| Moon | 1.62 | 113.4 | ~16.5% |
| Mars | 3.72 | 260.4 | ~38% |
| Jupiter | 24.8 | 1736 | ~253% |
| Sun (surface) | 274 | 19180 | ~2797% |
| Deep space (free fall) | 0 | 0 | 0% (weightless) |
Interactive Tools
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Related Terms
Mass
Mass is a fundamental scalar quantity that measures the amount of matter in an object and determines its resistance to acceleration (inertia). Unlike weight, mass does not depend on gravitational field strength and remains constant regardless of location in the universe. Mass is measured in kilograms (kg) in the SI system and plays a central role in Newton's Second Law, gravitational force calculations, and energy equations.
Gravity
Gravity is the fundamental attractive force that acts between any two objects with mass, pulling them toward each other. On the surface of the Earth, gravity gives all objects a downward acceleration of approximately 9.8 m/s², which determines their weight and governs projectile motion. Gravity keeps planets in orbit around the Sun, holds the atmosphere in place, and causes tides through the Moon's gravitational pull on Earth's oceans.
Normal Force
The normal force is the contact force exerted by a surface on an object, acting perpendicular (normal) to the surface at the point of contact. It is a reaction force that prevents objects from passing through solid surfaces and adjusts in magnitude to balance components of other forces. On a flat horizontal surface, the normal force on a stationary object equals its weight; on an inclined surface, it equals the component of weight perpendicular to the slope.
From Old English 'wihte' and Proto-Germanic 'wihtiz' meaning a measure. In physics, the term was formalised to mean gravitational force, distinguishing it from mass. The Newton (unit) is named after Sir Isaac Newton, and weight is expressed in Newtons in the SI system.