What is the difference between mass and weight?

Mass is the amount of matter in an object, measured in kilograms, and stays the same everywhere. Weight is the gravitational force on that mass, measured in newtons or pounds-force, and varies depending on gravitational acceleration. A 70 kg person weighs 687 N on Earth but only 113 N on the Moon.

How much would I weigh on the Moon?

You would weigh about 1/6 (16.5%) of your Earth weight on the Moon. The Moon's surface gravity is 1.62 m/s² compared to Earth's 9.81 m/s². A 150-pound person on Earth would weigh about 25 pounds on the Moon.

Why are astronauts weightless in space?

Astronauts in orbit experience apparent weightlessness because they're in continuous free fall toward Earth. The spacecraft and everything in it fall together at the same rate, so there's no relative force between them. They're not beyond Earth's gravity—at ISS altitude, gravity is still about 90% of surface strength.

Gravity and Weight

The Force That Keeps Us Grounded

Understand Gravity

You weigh less on the Moon but your mass stays the same. Weight is a force—specifically, the gravitational force between you and whatever planet or moon you're standing on. 理解する this relationship clarifies many everyday and scientific concepts.

Mass vs Weight

Mass (Measured in kg)

Amount of matter in an object
Same everywhere in the universe
A 70 kg person has 70 kg of mass on Earth, Moon, or Mars

Weight (Measured in N or lbf)

Gravitational force on an object
Varies with location (different gravitational acceleration)
A 70 kg person weighs 687 N on Earth, 114 N on Moon

Gravitational Acceleration Comparison

Location	g (m/s²)	g (relative to Earth)	70 kg person weighs
Earth (sea level)	9.81	1.00	687 N / 154 lbf
Moon	1.62	0.165	113 N / 25 lbf
Mars	3.72	0.38	260 N / 59 lbf
Jupiter	24.79	2.53	1,735 N / 390 lbf
ISS (orbit)	~0	~0	~0 (microgravity)

Convert Force Units

Why Gravity Varies

Newton's Law of Gravitation

F = G × (m₁ × m₂) / r²

Gravitational force depends on:

Masses: More massive objects attract more strongly
Distance: Force decreases with square of distance

On Planets

Surface gravity depends on:

Planet mass: More mass = stronger gravity
Planet radius: Larger radius = surface is farther from center

Jupiter is massive but also huge, so surface gravity is only 2.5× Earth's.

Calculating Weight

On Earth

A 70 kg person:

Weight = 70 kg × 9.81 m/s² = 686.7 N
In pounds-force: 686.7 ÷ 4.448 = 154.4 lbf

On the Moon

Same 70 kg person:

Weight = 70 kg × 1.62 m/s² = 113.4 N
In pounds-force: 113.4 ÷ 4.448 = 25.5 lbf

On Mars

Same 70 kg person:

Weight = 70 kg × 3.72 m/s² = 260.4 N
In pounds-force: 260.4 ÷ 4.448 = 58.5 lbf

Weightlessness

Free Fall

Astronauts in orbit aren't beyond Earth's gravity—they're in continuous free fall. The ISS and its contents fall toward Earth together, creating apparent weightlessness (microgravity).

True Zero Gravity

True weightlessness only exists very far from any mass. Even in deep space, every object exerts gravitational pull, though it may be immeasurably small.

Practical Applications

Scales and Weighing

Most scales actually measure force (weight), not mass. A spring scale calibrated on Earth would give wrong readings on the Moon—the mass hasn't changed, but the weight has.

Balance Scales

Balance scales compare masses and work correctly anywhere because both sides experience the same gravitational acceleration.

Engineering

Structural engineers must calculate gravitational forces (weight) on buildings, bridges, and vehicles to ensure adequate support.

まとめ

Weight is the gravitational force on an object, calculated as mass times gravitational acceleration (W = mg). While mass stays constant everywhere, weight varies with location. On Earth's surface, g ≈ 9.81 m/s²; on the Moon, it's about 1/6 as much. 理解する this distinction between mass and weight is fundamental to physics and engineering.

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