Do heavier objects experience a greater gravitational force?

Yes, a heavier object experiences a greater gravitational force because its weight is given by $W=mg$. Doubling the mass doubles the object's weight at the same location. However, doubling the mass also doubles its inertia, or resistance to acceleration. Newton's second law gives $a=F/m$, so the increased force is divided by an equally increased mass. The resulting free-fall acceleration is therefore still $g$, provided gravity is the only significant force.

Would a feather and a hammer land at the same time in a vacuum?

A feather and a hammer released together from the same height would land at the same time in a vacuum if they had the same initial velocity. Both would accelerate at the local gravitational acceleration because neither would experience air resistance. In ordinary air, the feather falls more slowly because drag is large compared with its weight. The hammer's compact shape and greater weight make drag less important to its motion. The vacuum changes the role of the surrounding fluid, not the gravitational force itself.

Is free-fall acceleration always exactly 9.81 m s⁻²?

No, 9.81 m s⁻² is a useful near-surface approximation rather than an exact value everywhere. Gravitational acceleration changes with distance from Earth's centre and also varies somewhat with latitude and local geology. At higher altitudes, its magnitude decreases because $g=GM/r^2$. IB questions normally provide an appropriate value or expect the standard near-surface approximation. Students should use the supplied value and state assumptions when the constant-acceleration model is relevant.

Does an object's shape matter when it falls in a vacuum?

Shape does not affect ideal free-fall acceleration when gravity is the only force acting. A sphere, sheet and irregular object at the same location all accelerate at the same rate in a vacuum. Shape becomes important in air because it affects cross-sectional area, airflow and drag. It can also matter if an extended object experiences a noticeably non-uniform gravitational field, although that is outside most basic IB problems. For ordinary near-surface vacuum questions, mass, shape and area do not alter the acceleration.

Can an object be in free fall while moving upward?

Yes, free fall is defined by the forces acting rather than the direction of motion. After a ball is thrown upward and leaves the hand, it is in free fall if air resistance is negligible. Its velocity initially points upward, while its acceleration points downward throughout the flight. At the highest point, its velocity is momentarily zero but its acceleration remains approximately $g$ downward. It then gains downward velocity under the same gravitational acceleration.

How should I explain equal free-fall acceleration in an IB Physics exam?

Begin by stating that the falling object's weight is $Fg=mg$. Apply Newton's second law in the form $a=F{\text{net}}/m$. In a vacuum, gravity is the only force, so substitute $F{\text{net}}=mg$ to obtain $a=mg/m=g$. Conclude explicitly that mass cancels and therefore does not affect free-fall acceleration. Include the condition that the objects are in the same gravitational field and that air resistance is absent or negligible.

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Why do heavier objects not fall faster in a vacuum?

• 29 Jul 2026•16

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Why do heavier objects not fall faster in a vacuum?

• 29 Jul 2026•16

A heavier object does experience a larger gravitational force than a lighter object. However, it also has proportionally greater inertia, meaning it is proportionally harder to accelerate. These two effects cancel exactly, so objects at the same location in a gravitational field have the same free-fall acceleration, provided gravity is the only significant force acting.

Near Earth's surface, this acceleration has a magnitude of approximately 9.81 m s⁻². Therefore, two objects released from the same height with the same initial velocity will have the same velocity and position at every later time in an ideal vacuum. Mass affects their weights, but not their accelerations.

This distinction is central to IB Physics free fall. It connects Newton's laws, gravitational fields, kinematics, force diagrams and the interpretation of motion graphs.

The direct explanation: gravitational force and inertia increase together

Suppose two objects have masses of 1 kg and 10 kg. Near Earth's surface, their weights are

W=mg.

Taking $g=9.81\text{ m s}^{-2}$ :

the 1 kg object has a weight of 9.81 N;
the 10 kg object has a weight of 98.1 N.

The heavier object therefore experiences ten times as much gravitational force. That fact sometimes leads students to conclude that it should accelerate ten times as quickly, but Newton's second law does not say that greater force alone means greater acceleration. It states

F_{\text{net}}=ma,

so acceleration depends on the ratio of resultant force to mass:

a=\frac{F_{\text{net}}}{m}.

For an object falling in a vacuum, the only significant force is its weight. Substituting $F_{\text{net}}=mg$ gives

a=\frac{mg}{m}=g.

The object's mass cancels. The 10 kg object experiences ten times the force, but it also has ten times the inertia. Both objects consequently accelerate at $g$ .

Object	Mass	Weight near Earth	Acceleration
Light object	1.0 kg	9.81 N	9.81 m s⁻² downward
Heavy object	10 kg	98.1 N	9.81 m s⁻² downward

This is the clearest Newtonian answer to why objects fall at the same rate. The cancellation is not a claim that mass is irrelevant to gravity. Mass increases both the gravitational force and the resistance to acceleration in the same proportion.

Gravitational mass and inertial mass

A deeper explanation distinguishes two roles played by mass.

Gravitational mass determines how strongly an object interacts with a gravitational field.
Inertial mass measures an object's resistance to acceleration when a resultant force acts.

Writing these separately gives

F_g=m_g g

and

F_{\text{net}}=m_i a.

For pure free fall,

m_i a=m_g g,

a=\frac{m_g}{m_i}g.

Experiments find gravitational and inertial mass to be equivalent. Consequently, the ratio $m_g/m_i$ is the same for all ordinary test bodies, producing the same gravitational acceleration. This universality of free fall helped motivate Einstein's equivalence principle, although IB calculations at this level are usually handled using Newtonian mechanics.

Modern tests continue to find no measurable composition-dependent difference in free-fall acceleration. The MICROSCOPE satellite mission, discussed by the American Physical Society's Physics publication, compared test masses of different compositions and found no violation of the equivalence principle within the experiment's sensitivity.

The gravitational-field derivation

The same conclusion follows from Newton's law of gravitation. For an object of mass $m$ at distance $r$ from the centre of a planet of mass $M$ , the gravitational force is

F_g=\frac{GMm}{r^2}.

Applying Newton's second law,

ma=\frac{GMm}{r^2}.

Cancelling $m$ gives

a=\frac{GM}{r^2}.

The acceleration depends on:

the mass $M$ of the planet producing the field;
the distance $r$ from the planet's centre;
the gravitational constant $G$ .

It does not depend on the mass of the falling test object. This equation also shows why $g$ should not always be treated as exactly 9.81 m s⁻². As altitude increases, $r$ increases and gravitational acceleration decreases.

The value 9.80665 m s⁻² is the defined standard acceleration of gravity reported by NIST's CODATA reference. It is a conventional standard, not the exact gravitational acceleration at every point on Earth. In IB questions, use the value supplied in the question or data booklet and avoid replacing it unnecessarily with a memorized value.

What “fall at the same rate” actually means

The phrase can be imprecise. Objects in the same local gravitational field have the same acceleration, not automatically the same velocity under every possible condition.

If two objects are released from the same position at the same time with the same initial velocity, equal acceleration means they will maintain equal velocities and positions. For constant acceleration,

v=u+gt

and

s=ut+\frac{1}{2}gt^2,

where downward has been chosen as positive. Neither expression contains the object's mass.

However, if one object is thrown downward while another is released from rest, their velocities will differ because their initial velocities differ. They still undergo the same gravitational acceleration. Similarly, objects dropped from substantially different altitudes may experience slightly different values of $g$ .

For exam purposes, a precise statement is:

In a vacuum, objects at the same location have the same acceleration due to gravity, independent of their mass. If they also have the same initial position and velocity, they follow the same motion.

Why objects appear to fall differently in air

Daily observations usually involve air rather than a vacuum. A falling object in air experiences at least two forces:

weight downward;
drag, or fluid resistance, upward relative to its motion.

Its resultant force is then

F_{\text{net}}=mg-F_D,

so its downward acceleration is

a=g-\frac{F_D}{m}.

Drag depends on factors such as speed, shape, cross-sectional area and fluid density. It is not generally proportional to mass, so it does not cancel in the same way as weight. A flat sheet of paper can therefore fall more slowly than a compact ball, even if their masses are similar.

Situation	Forces acting	Acceleration
Vacuum near Earth's surface	Weight only	Approximately constant at $g$
Air at low initial speed	Weight and relatively small drag	Downward, but usually less than $g$
Air as speed increases	Weight and increasing drag	Decreases in magnitude
Terminal speed	Drag balances weight	Zero

A useful demonstration is to compare a flat sheet of paper with the same sheet crumpled into a compact ball. Their masses are unchanged, but the crumpled paper usually falls faster because its orientation and effective area produce less drag. This shows that the difference is caused by interaction with the air, not by a different gravitational acceleration.

For a fuller treatment of motion in fluids, see RevisionDojo's explanation of terminal velocity in IB Physics. It explains how increasing drag eventually balances weight, making the resultant force and acceleration zero.

Free fall does not necessarily mean moving downward

In physics, free fall means motion under the influence of gravity alone. It does not require the object to be travelling downward.

A ball thrown vertically upward is in free fall after leaving the thrower's hand if air resistance is neglected. Its velocity is upward initially, but its acceleration remains downward throughout the motion. At the highest point, its instantaneous velocity is zero while its acceleration is still $g$ downward.

An orbiting satellite is also in continuous free fall. Gravity accelerates it toward the planet, while its tangential velocity causes it continually to miss the surface. Free fall is therefore defined by the forces acting, not by the direction of motion or by whether the object eventually hits the ground.

These ideas sit within the current IB Physics theme A: Space, time and motion, including kinematics and forces. The official IB Physics subject brief outlines the course structure for the syllabus first assessed in 2025, while the IB Physics curriculum update explains the revised assessment model. RevisionDojo's topic-wide IB Physics Kinematics Explained for Exams provides the broader exam-focused coverage of motion graphs, constant-acceleration equations and projectile motion.

How the motion appears on graphs

For ideal free fall near Earth's surface, the acceleration is treated as constant over modest vertical distances. The graph shapes follow directly from this model.

Acceleration-time graph

If downward is positive, the graph is a horizontal line at $+g$ . If upward is positive, it is a horizontal line at (-g). Mass does not change the line.

Velocity-time graph

The velocity changes linearly with time. Its gradient is the gravitational acceleration, so two objects released with the same initial velocity have overlapping velocity-time graphs.

Position-time graph

Position changes quadratically with time because

s=ut+\frac{1}{2}at^2.

For an object released from rest, the distance fallen is proportional to $t^2$ , not to $t$ . Doubling the fall time therefore produces four times the displacement under the constant- $g$ model.

Further graph interpretation and equation selection are covered in RevisionDojo's IB Physics mechanics resources. The IB Physics data booklet resource is also useful for checking notation and relevant equations.

Worked IB Physics example

A 0.20 kg ball and a 5.0 kg metal object are released simultaneously from rest in a vacuum chamber. They fall through a vertical distance of 12 m. Assume $g=9.81\text{ m s}^{-2}$ .

For either object,

s=ut+\frac{1}{2}gt^2.

Since $u=0$ ,

12=\frac{1}{2}(9.81)t^2.

Therefore,

t=\sqrt{\frac{24}{9.81}}=1.56\text{ s}.

The final speed is

v=u+gt=(9.81)(1.56)=15.3\text{ m s}^{-1}.

Both objects take 1.56 s and reach a speed of 15.3 m s⁻¹. Their weights differ, but mass does not appear in either kinematics calculation because both have acceleration $g$ .

To connect the concept to exam-style applications, students can use the A.1 Kinematics Questionbank. Focus on explaining the force-to-mass ratio rather than merely memorizing that all objects fall together.

Common misconceptions to avoid

“The heavier object has more gravity, so it accelerates faster”

The first clause is partly correct: the heavier object experiences a larger gravitational force. The conclusion is incorrect because its inertial mass is larger by the same factor. Acceleration depends on (F/m), not on force alone.

“Gravity pulls every object with the same force”

This is false. Weight is (mg), so objects of different mass experience different gravitational forces in the same field. What is equal is their acceleration when gravity is the only significant force.

“A vacuum removes gravity”

A vacuum is a region with negligible matter, particularly negligible gas. It removes air resistance but does not remove a gravitational field. An evacuated chamber on Earth still lies within Earth's gravitational field.

“At the highest point, acceleration is zero”

For a vertically thrown object, velocity is momentarily zero at the top. Its acceleration remains downward at approximately $g$ . Confusing velocity with acceleration is a frequent IB error.

“The value of g is exactly the same everywhere”

Local gravitational acceleration varies with altitude, latitude and the distribution of mass around Earth. The constant value used in short near-surface problems is a model. A strong answer states the relevant assumption rather than presenting $g$ as universally constant.

Exam technique for explaining equal free-fall acceleration

When an IB question asks why masses fall with the same acceleration, structure the response as a short causal argument:

State that the gravitational force is proportional to mass: $F_g=mg$ .
Apply Newton's second law: $a=F_{\text{net}}/m$ .

Include the condition that air resistance is absent or negligible. If a question says an object is in free fall, check whether the wording defines gravity as the only force. Do not add drag to a model that explicitly neglects it.

Use signs consistently in calculations. If upward is positive, write $a=-g$ ; if downward is positive, write $a=+g$ . The value of $g$ is a magnitude, while acceleration is a vector with direction.

NASA's free-falling objects explanation presents the same cancellation through $W=mg$ and $F=ma$ . The OpenStax free-fall chapter also provides worked kinematics examples and emphasizes the need for a consistent sign convention.

Limits of the simple model

The usual IB model treats the falling body as a test object whose gravitational effect on Earth is negligible. Strictly speaking, the object pulls Earth upward while Earth pulls the object downward with an equal force. Earth's acceleration is normally immeasurably small in classroom situations because its mass is enormous.

The approximation of constant $g$ also works only when the change in height is small compared with Earth's radius. Over very large distances, use

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

and recognize that acceleration changes with position. Objects at different distances from Earth's centre need not have identical accelerations even in a vacuum.

For extended bodies in a non-uniform field, different parts can experience slightly different gravitational accelerations. These differences produce tidal effects. None of these qualifications changes the standard conclusion for two ordinary objects released side by side in a vacuum near Earth's surface.

Conclusion

Heavier objects do not fall faster in a vacuum because their greater gravitational force is matched by proportionally greater inertia. Applying $F=ma$ and $W=mg$ gives $a=g$ , so the mass of the falling object cancels. Equal acceleration produces identical motion only when the objects also share the same initial conditions and experience the same local gravitational field.

In air, objects can fall differently because drag depends on shape, area and speed rather than simply scaling with mass. For IB Physics revision, practise stating the model, drawing the forces, applying Newton's second law and maintaining a consistent sign convention. RevisionDojo's kinematics notes, Jojo AI feedback and targeted Questionbank problems can help turn this explanation into reliable exam technique.

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