Why does mass stay the same on the Moon while weight decreases?

Mass stays the same because it measures the object's inertia, which does not depend on location. Moving an object to the Moon does not change the object itself, so its mass remains constant. Weight decreases because the Moon's gravitational field strength is only about $1.6\text{ N kg}^{-1}$, compared with approximately $9.8\text{ N kg}^{-1}$ near Earth's surface. Since $W = mg$, the smaller value of $g$ produces a smaller gravitational force. An object consequently weighs about one-sixth as much on the Moon as it does on Earth. Its resistance to acceleration nevertheless remains associated with the same mass.

Is weight measured in kilograms or newtons?

In physics, weight is measured in newtons because weight is a force. Mass is measured in kilograms, which is why an IB answer should not describe a weight as a certain number of kilograms. Everyday language often uses “weight” to mean mass, especially on bathroom scales and product labels. A scale marked in kilograms measures a supporting force and converts it into a mass reading using an assumed gravitational field strength. For example, a 10 kg mass has a weight of about 98 N near Earth's surface. Keeping the units distinct is one of the simplest ways to avoid mass-versus-weight errors.

Can an object have mass but no weight?

An object can have mass while experiencing zero or negligible gravitational weight in an idealized situation where the local gravitational field is zero. Its inertia remains, so a force would still be needed to accelerate it. In practice, gravitational fields extend through space and do not end at a sharp boundary, making exactly zero gravitational force difficult to achieve. An astronaut in orbit is not an example of gravity disappearing because Earth's gravity continues to provide the orbital acceleration. The astronaut instead has approximately zero apparent weight because there is almost no supporting normal force. IB answers should therefore distinguish zero apparent weight from zero gravitational force.

Does a heavier object fall faster than a lighter object?

In the absence of air resistance, objects at the same location have the same free-fall acceleration regardless of mass. A more massive object has a greater weight because $W = mg$, but it also has proportionally greater inertia. Applying $F = ma$ gives $mg = ma$, so the object's mass cancels and $a = g$. This is why a heavy object and a light object fall together in a vacuum. In air, differences in drag, shape, area, and mass can produce different observed accelerations. A falling feather and ball therefore behave differently in air but not because gravity gives the feather a fundamentally smaller free-fall acceleration.

Why do scales show different readings in an accelerating lift?

A scale measures the normal contact force between the scale and the person, not mass directly. When the lift accelerates upward, the normal force must exceed weight, so the scale reading increases. When the lift accelerates downward, the normal force is smaller than weight, so the reading decreases. The relationship is found from $N - mg = ma$ when upward is chosen as positive. The person's mass remains constant, and their gravitational weight is approximately unchanged over the lift's small change in altitude. The changing reading is therefore described as a change in apparent weight.

How can an IB Physics graph show the relationship between mass and weight?

Plot weight on the vertical axis and mass on the horizontal axis. From $W = mg$, the expected graph is a straight line through the origin when gravitational field strength is constant. The gradient equals $g$ because $W/m = g$. A gradient of $9.8\text{ N kg}^{-1}$ is consistent with measurements near Earth's surface. A non-zero intercept may indicate a zero error, an additional force, or imperfect calibration rather than a new mass-weight law. In an exam, students should state the gradient's physical meaning and include its correct unit.

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What is the difference between mass and weight?

• 29 Jul 2026•16

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What is the difference between mass and weight?

• 29 Jul 2026•16

Mass and weight are related, but they are not the same physical quantity. Mass measures an object's inertia and is measured in kilograms, while weight is the gravitational force acting on that mass and is measured in newtons. For an unchanged object, mass remains constant when it moves between planets, but weight changes because gravitational field strength changes.

This distinction is central to mass vs weight physics questions in IB Physics mechanics. It affects force calculations, free-body diagrams, gravitational fields, experimental graphs, and explanations of apparent weightlessness.

What is the difference between mass and weight?

The clearest comparison is:

Property	Mass	Weight
Meaning	Measure of an object's inertia	Gravitational force acting on an object
Common symbol	$m$	$W$ or $F_g$
SI unit	kilogram, kg	newton, N
Scalar or vector?	Scalar	Vector
Depends on location?	No, provided the object itself does not change	Yes, because gravitational field strength varies
Relevant equation	Appears in $F = ma$	$W = mg$
Typical measuring instrument	Balance	Force meter or calibrated spring scale

In everyday speech, people often say that someone “weighs 60 kg.” Scientifically, this means the person has a mass of 60 kg. Their weight near Earth's surface is approximately $60 \times 9.8 = 588\text{ N}$ .

This is not merely a vocabulary issue. In an IB Physics calculation, writing kilograms as the unit of weight or newtons as the unit of mass indicates that two different physical quantities have been confused.

What is mass in physics?

Mass is a scalar quantity that measures an object's resistance to acceleration. This property is called inertia: a larger mass requires a larger resultant force to produce the same acceleration.

Newton's second law shows this relationship:

$F_{net} = ma$

If the same resultant force acts on objects of 2 kg and 4 kg, the 2 kg object has twice the acceleration of the 4 kg object. The larger mass has greater inertia, so its motion is more difficult to change.

Mass also determines how strongly an object participates in gravitational interactions. In introductory mechanics, inertial mass in $F = ma$ and gravitational mass in the gravitational-force equation are treated as equivalent. IB students do not normally need to distinguish experimentally between them, but recognizing both roles helps explain why mass appears in equations for inertia and gravity.

Why is mass constant?

An object's mass is an intrinsic property of that object. Moving a 5 kg object from Earth to the Moon does not remove matter from it or change its inertia, so it remains a 5 kg object.

The statement that mass is constant assumes the object itself remains unchanged. If fuel leaves a rocket, material is added to a container, or energy is transferred in a context where mass-energy changes are significant, the system's mass can change. For ordinary IB physics mechanics problems involving objects transported between planets, however, the expected conclusion is that mass does not depend on gravitational field strength.

Mass is therefore not “the force of gravity in kilograms.” The kilogram is the SI base unit of mass, as confirmed by the NIST explanation of SI mass.

What is weight in physics?

Weight is the gravitational force exerted on an object. In the current IB Physics course, gravitational force near a planet is represented by:

$F_g = mg$

It is also common to write:

$W = mg$

where:

$W$ or $F_g$ is weight in newtons
$m$ is mass in kilograms
$g$ is gravitational field strength in newtons per kilogram

Weight is a vector because force has both magnitude and direction. Near the surface of a spherical planet, weight acts toward the planet's centre.

The current IB Physics course places forces within A.2 Forces and momentum and gravitational fields within D.1 Gravitational fields. The official IB Physics subject brief confirms the current course structure, while the IB Physics curriculum update confirms that it was first assessed in May 2025.

For wider mechanics context, use RevisionDojo's IB Physics mechanics resources and A.2 Forces and Momentum notes. This article focuses specifically on mass and weight rather than repeating the complete mechanics topic.

Why does weight change with gravity?

The equation $W = mg$ shows that weight depends on two quantities. The object's mass $m$ remains unchanged, but the local gravitational field strength $g$ can vary.

Gravitational field strength is defined as gravitational force per unit mass:

$g = \frac{F_g}{m}$

Its unit is $\text{N kg}^{-1}$ . Since $1\text{ N} = 1\text{ kg m s}^{-2}$ , this unit is dimensionally equivalent to $\text{m s}^{-2}$ . Consequently, gravitational field strength and free-fall acceleration have the same numerical value when gravity is the only relevant force.

For a spherical planet or a point mass:

$g = \frac{GM}{r^2}$

Here, $M$ is the planet's mass and $r$ is the distance from its centre. Combining this with $W = mg$ gives:

$W = \frac{GMm}{r^2}$

Weight can therefore change because:

the attracting planet has a different mass
the planet has a different radius
the object moves farther from or closer to the planet's centre
effective gravity changes slightly because of planetary rotation and location

A planet with more mass does not automatically have proportionally greater surface gravity. Its radius also matters because $g$ depends on $1/r^2$ . This explains why surface-gravity comparisons cannot be made from planetary mass alone.

RevisionDojo's IB Physics circular motion and gravitation explained guide provides the broader exam-focused treatment of gravitational fields, inverse-square relationships, and orbits. The D.1 Gravitational Fields notes are useful when connecting $W = mg$ to $g = GM/r^2$ .

How do mass and weight compare on different planets?

Consider an astronaut with a mass of 60 kg. The mass remains 60 kg at each location, but multiplying by the local value of $g$ gives a different weight.

Location	Approximate $g$	Mass	Weight using $W = mg$
Moon	$1.6\text{ N kg}^{-1}$	60 kg	96 N
Mars

These approximate gravitational values come from NASA's Planetary Fact Sheet. For gas giants such as Jupiter, “surface gravity” is quoted at a defined atmospheric pressure level because there is no solid surface on which a person could stand.

The astronaut would weigh about 38% as much on Mars as on Earth. On the Moon, the astronaut's weight would be roughly one-sixth of the Earth value. On Jupiter's reference level, it would be more than twice the Earth value.

The astronaut's inertia would nevertheless remain the same. Pushing the astronaut sideways on the Moon would not feel six times easier in the idealized sense of producing acceleration from a given horizontal resultant force. The astronaut has less weight but still has a mass of 60 kg.

How are mass and weight measured?

A balance compares masses. A traditional beam balance compares an unknown object with reference masses, so the same gravitational field acts on both sides and its effect cancels from the comparison.

A spring scale or force meter measures force through the extension of a spring. If it is calibrated in newtons, it measures weight directly when held stationary in a gravitational field. If it displays kilograms, it is converting the measured force into an estimated mass using an assumed value of $g$ .

This is why an Earth-calibrated bathroom scale would give a misleading mass reading on the Moon unless recalibrated. It experiences a smaller force from the same person and interprets that smaller force using Earth's gravitational field strength.

IB experimental questions may present a graph of weight against mass. Since $W = mg$ , a graph with weight on the vertical axis and mass on the horizontal axis should be a straight line through the origin, with:

$\text{gradient} = g$

A gradient of $9.8\text{ N kg}^{-1}$ therefore indicates a gravitational field strength of approximately $9.8\text{ N kg}^{-1}$ . Students can practise this type of interpretation in the D.1 Gravitational Fields Questionbank.

Weight, normal force, and apparent weight

A scale does not usually measure gravitational force directly. It measures the normal contact force that the scale exerts on the person, and then reports a corresponding reading.

For a person standing at rest on a horizontal floor:

$N = mg$

The normal force and weight have equal magnitudes because the resultant vertical force is zero. They are still separate forces arising from different interactions: weight comes from gravity, while the normal force comes from contact with the floor.

In an accelerating lift, the forces are not necessarily equal. Taking upward as positive:

$N - mg = ma$

If the lift accelerates upward, $N > mg$ and the person feels heavier. If it accelerates downward, $N < mg$ and the person feels lighter. The person's mass and gravitational weight have not suddenly changed significantly; the scale reading has changed because the supporting force is different.

This scale reading is often called apparent weight. In a free-falling lift, both the person and the scale accelerate downward together, so the normal force can become zero even though gravity continues to act.

Are astronauts in orbit weightless?

Astronauts in orbit appear weightless because they and their spacecraft are in continuous free fall around Earth. Gravity has not disappeared. In fact, gravity is the force producing the centripetal acceleration required for the orbit.

An orbiting astronaut's gravitational weight, understood as the gravitational force $F_g$ , is therefore not zero. It is somewhat smaller than at Earth's surface because the astronaut is farther from Earth's centre, but it remains substantial in low Earth orbit.

What becomes nearly zero is the astronaut's apparent weight because there is no ordinary supporting normal force. NASA's mass versus weight activities use space-station demonstrations to clarify this distinction.

In an exam response, avoid writing “there is no gravity in space.” A stronger explanation is: the astronaut and spacecraft accelerate together under gravity, so no supporting contact force is required and the astronaut experiences apparent weightlessness.

How to solve mass and weight questions in IB Physics

A reliable method is:

Identify whether the question gives mass, weight, or a scale reading.
Convert mass to kilograms if necessary.
Identify the appropriate local value of $g$ .
Use $W = mg$ for gravitational weight near the stated location.
Draw a free-body diagram if other forces are present.
Apply $\Sigma F = ma$ rather than assuming the normal force equals weight.
Give mass in kilograms and force in newtons.

Worked example: finding weight on Mars

A scientific instrument has a mass of 12 kg. Find its weight on Mars, where $g = 3.7\text{ N kg}^{-1}$ .

$W = mg = 12 \times 3.7 = 44.4\text{ N}$

The instrument's weight is 44 N to two significant figures. Its mass remains 12 kg.

Worked example: finding mass from weight

An object has a weight of 147 N near Earth's surface, where $g = 9.8\text{ N kg}^{-1}$ .

$m = \frac{W}{g} = \frac{147}{9.8} = 15\text{ kg}$

The answer is a mass, so the correct unit is kilograms. The relevant equations and constants can be checked in RevisionDojo's IB Physics data booklet resource.

Common mass vs weight exam mistakes

Using the wrong units

Mass is measured in kg, while weight is measured in N. A statement such as “the weight is 12 kg” is not scientifically correct in an IB Physics response.

Treating $g$ as universal

The value $9.8\text{ N kg}^{-1}$ applies approximately near Earth's surface. Use the value supplied in the question when working on another planet or at a substantial altitude.

Saying mass is the amount of gravity

Mass describes inertia and gravitational interaction; it is not a measure of local gravitational pull. Weight is the quantity that depends directly on the local field.

Assuming normal force always equals weight

The equality $N = mg$ only follows when the vertical resultant force is zero and no other vertical forces alter the balance. It is not automatically true in lifts, on inclined planes, or during accelerated motion.

Adding weight and gravity as separate forces

Weight is the gravitational force. A free-body diagram should not contain one arrow labelled “weight” and another separate arrow labelled “gravity” for the same interaction.

Confusing $g$ and $G$

Lowercase $g$ is local gravitational field strength, measured in $\text{N kg}^{-1}$ . Uppercase $G$ is the universal gravitational constant used in $F = GMm/r^2$ .

Conclusion

The essential difference between mass and weight is that mass measures inertia, while weight is gravitational force. Mass remains constant when an unchanged object moves between planets, but weight changes because the local gravitational field strength changes.

For IB Physics exams, remember $W = mg$ , use kilograms for mass and newtons for weight, and distinguish gravitational weight from a scale's normal-force reading. After reviewing the concept, use RevisionDojo's gravitational-fields Questionbank and ask Jojo AI to diagnose errors in units, force diagrams, or written explanations.

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