Is terminal velocity the fastest speed an object can ever reach?

Terminal velocity is the constant speed produced by a particular force balance, not an absolute speed limit for the object. Its value depends on the object's mass, shape, area, orientation, and surrounding fluid. If the conditions change, the same object can have a different terminal velocity. For example, a skydiver has a lower terminal speed after opening a parachute because the area and drag increase. An additional driving force could also make the object move faster than its gravity-driven terminal speed, although drag would then increase accordingly.

Why is acceleration zero at terminal velocity if gravity still acts?

Gravity still acts and gives the object a downward weight at terminal velocity. However, the surrounding fluid exerts an equal upward drag force. Because these forces have equal magnitudes and opposite directions, their vector sum is zero. Newton's second law then gives zero acceleration because $a=F{net}/m$. The object continues moving because zero resultant force preserves constant velocity rather than forcing velocity to zero.

Do heavier objects have a greater terminal velocity?

A heavier object has a greater terminal velocity only when relevant factors such as shape, projected area, and drag coefficient are held constant. Its greater weight requires a larger drag force before equilibrium can occur. Because drag increases with speed, the object must then fall faster to generate that larger drag. However, a heavier object with a very large area may have a lower terminal speed than a lighter, compact object. IB answers should therefore compare weight with drag-producing properties rather than claiming that heavier objects always fall faster.

Why does opening a parachute reduce terminal velocity?

Opening a parachute greatly increases the projected area exposed to the airflow and usually increases the overall drag coefficient. At the skydiver's original speed, the new drag force can temporarily become greater than weight. This produces an upward resultant force and an upward acceleration, meaning the downward velocity decreases. As the skydiver slows, drag decreases until it once again equals weight. The resulting force balance establishes a new, much lower terminal velocity.

Can terminal velocity occur in water or other liquids?

Terminal velocity can occur in any fluid, including air, water, and oil. In a liquid, drag may become significant at a much lower speed because liquids are generally denser than air. Buoyancy may also be large enough to include explicitly in the force balance. A sinking object reaches terminal velocity when its weight is balanced by upward drag and buoyancy together. For small spheres moving slowly through viscous liquids, Stokes' law may provide a more suitable drag model than quadratic drag.

Does a falling object always reach terminal velocity before hitting the ground?

A falling object does not always have enough time or distance to reach terminal velocity. It must first accelerate until its speed produces enough drag to balance the relevant driving force. During a short fall, impact may occur while the object is still accelerating. In ideal drag models, terminal speed is often approached asymptotically rather than reached exactly in finite time. In practical measurements and IB questions, the velocity is treated as terminal once it becomes effectively constant within the precision of the data.

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Why does terminal velocity exist?

• 29 Jul 2026•16

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Why does terminal velocity exist?

• 29 Jul 2026•16

Terminal velocity exists because drag increases as an object moves faster through a fluid. During a fall, the increasing upward drag eventually balances the object's downward weight, making the resultant force and acceleration zero. The object then continues falling at a constant velocity rather than becoming stationary.

For IB Physics, the central chain of reasoning is:

speed increases → drag increases → resultant force decreases → acceleration decreases → drag balances weight → terminal velocity

This explanation connects kinematics with Newton's laws and force diagrams. It also explains why terminal velocity varies with mass, shape, cross-sectional area, orientation, and the properties of the surrounding fluid.

What is terminal velocity?

Terminal velocity is the constant velocity reached by an object moving through a fluid when the forces acting along its direction of motion are balanced. For an object falling through air, the main forces are usually:

Weight, $W = mg$ , acting downward
Air resistance or drag, $F_D$ , acting upward against the motion

At terminal velocity:

$F_D = mg$

Therefore:

$F_{net} = mg - F_D = 0$

Newton's second law gives:

$F_{net} = ma$

Because the resultant force is zero, the acceleration is also zero. Zero acceleration does not mean zero velocity. It means the velocity is no longer changing, so the object continues downward at a constant speed.

Although the phrase terminal velocity is widely used, terminal speed is sometimes more precise because the defining value is the magnitude of the steady velocity. In a one-dimensional vertical fall, either term is normally understood.

Why does terminal velocity happen?

Consider an object released from rest in still air. Its motion develops in several stages rather than changing instantly from rest to terminal velocity.

Stage of fall	Weight	Drag	Resultant force	Acceleration
Immediately after release	$mg$ downward	Zero or negligible	Approximately $mg$ downward	Approximately $g$ downward
Object speeding up	$mg$ downward	Increasing upward	Downward but decreasing	Downward but decreasing
Terminal velocity	downward

Immediately after release

At the instant of release, the object's speed relative to the air is zero. Its drag is therefore zero or negligible, although its weight is already acting downward.

The initial resultant force is approximately $mg$ , so:

$a \approx g$

The object begins accelerating downward. This is only approximately free fall because true free fall means that gravity is the only force acting.

As the object becomes faster

Once the object moves through the air, it experiences drag opposite to its velocity. Faster motion generally produces greater drag because the object must displace air more rapidly and transfers more momentum to the surrounding fluid.

Taking downward as positive, Newton's second law can be written as:

$mg - F_D = ma$

As $F_D$ increases, the difference $mg-F_D$ becomes smaller. The acceleration therefore decreases even though the object is still speeding up.

This is an important IB distinction: decreasing acceleration does not necessarily mean decreasing speed. While the acceleration remains downward, the downward speed continues to increase, but it increases more slowly.

When the forces balance

Eventually, the drag becomes equal in magnitude to the object's weight. The forces then act in opposite directions with equal magnitudes:

$F_D = mg$

The resultant force becomes zero, so the acceleration becomes zero. Newton's first law then predicts that the object continues moving at constant velocity.

This constant downward velocity is its terminal velocity under those particular conditions. It is not a universal speed attached permanently to the object.

How drag creates a limiting speed

A limiting speed can exist because drag depends on the object's speed relative to the fluid. Weight near Earth's surface remains approximately constant during an ordinary fall, but drag grows as the object becomes faster.

For many objects moving at moderate or high speeds through air, drag is modeled by the quadratic drag equation:

$F_D = \frac{1}{2}C_D\rho A v^2$

Here:

$C_D$ is the dimensionless drag coefficient, influenced by shape, orientation, and flow conditions
$\rho$ is the density of the fluid
$A$ is the reference or projected area perpendicular to the flow
$v$ is the object's speed relative to the fluid

The NASA drag equation shows why speed has such a strong effect. If the speed doubles while the other variables remain approximately constant, quadratic drag becomes four times as large.

At terminal velocity, drag equals weight:

$mg = \frac{1}{2}C_D\rho A v_t^2$

Rearranging gives:

$v_t = \sqrt{\frac{2mg}{C_D\rho A}}$

This equation is a useful model for relatively large objects moving through air. It should not be treated as the only possible drag law, because drag behavior depends on the flow regime.

Why terminal velocity varies by object

The quadratic model shows that terminal velocity depends on the balance between weight and the object's ability to produce drag.

Change, with other factors fixed	Effect on terminal velocity	Reason
Greater mass $m$	Increases	More drag is needed to balance the larger weight
Greater projected area $A$	Decreases	The object interacts with more fluid at a given speed
Greater drag coefficient $C_D$	Decreases	The shape or orientation produces more drag
Greater fluid density $\rho$

Mass and area must be considered together

It is incomplete to say that heavier objects always have higher terminal velocities. The equation predicts a higher terminal velocity when mass increases while area, shape, and fluid conditions remain unchanged.

A large sheet of paper can have more mass than a compact ball but still fall more slowly because its projected area and drag coefficient are much larger. Crumpling the same paper leaves its mass nearly unchanged while reducing its area and changing its shape, so its terminal velocity increases.

A useful comparison is the ratio $m/A$ . An object with a large mass relative to its projected area generally needs a higher speed before drag can balance its weight.

Shape and orientation matter

The drag coefficient represents how strongly an object's geometry and orientation affect the flow. A broad, blunt shape usually produces more drag than a streamlined shape with the same reference area.

A skydiver can therefore change terminal velocity without changing mass. A spread-out position increases projected area and typically increases drag, producing a lower terminal speed. A compact, head-down position reduces the effective area and can produce a higher terminal speed.

A parachute applies the same principle more dramatically. Opening it greatly increases area and changes the drag coefficient, creating a new terminal velocity that is much lower than the original one.

The surrounding fluid matters

Air is a fluid, but terminal motion can also occur in water, oil, or other liquids. A denser or more viscous fluid can provide substantial resistance at a much lower speed.

The relevant speed is the object's speed relative to the fluid, not necessarily relative to the ground. A falling object in an updraft can have a different ground velocity from its velocity relative to the surrounding air.

Linear drag and Stokes' law

Quadratic drag is appropriate for many familiar objects falling through air, but small spheres moving slowly through a viscous fluid may experience approximately linear drag. Under suitable laminar-flow conditions, Stokes' law gives:

$F_D = 6\pi\eta r v$

Here, $\eta$ is the fluid's dynamic viscosity, $r$ is the sphere's radius, and $v$ is its speed relative to the fluid. If buoyancy is neglected, terminal speed follows from:

$mg = 6\pi\eta r v_t$

The important concept remains unchanged: a speed-dependent resistive force grows until it balances the driving force. The mathematical relationship between drag and speed determines the numerical terminal velocity and how quickly it is approached.

In a liquid, buoyancy may not be negligible. The complete force balance for an object sinking at terminal velocity can then be written as:

$W = F_B + F_D$

where $F_B$ is the upward buoyant force. For most ordinary skydiver questions, air buoyancy is small compared with weight and is omitted unless the problem states otherwise.

What the motion graphs look like

Terminal-velocity questions often appear through graphs rather than direct calculations. You should be able to connect each graph to the changing forces.

Velocity-time graph

Starting from rest, the velocity increases rapidly at first. The gradient then becomes progressively smaller because the acceleration is decreasing.

The graph eventually becomes horizontal, or approaches a horizontal line. That horizontal value represents terminal velocity because the gradient of a velocity-time graph is acceleration.

Acceleration-time graph

The initial acceleration is approximately $g$ downward if drag is initially negligible. As drag increases, the acceleration's magnitude decreases toward zero.

The acceleration does not become negative merely because it is getting smaller. Its sign depends on the chosen positive direction and whether the object is speeding up or slowing down.

Drag-time graph

Drag begins at zero or a small value and increases as speed increases. It approaches the value $mg$ for a fall through air when buoyancy is neglected.

Weight remains approximately constant throughout the fall. A common mistake is to draw weight decreasing as drag increases, but the two forces change independently.

Does an object reach terminal velocity immediately?

No. The object needs time and distance to accelerate until drag becomes comparable with its weight. A short fall may end before the object comes close to terminal velocity.

In ideal mathematical models, the speed often approaches terminal velocity asymptotically. This means it gets progressively closer without becoming exactly equal in a finite time. In experimental work and exam questions, the object is treated as having reached terminal velocity once its speed is effectively constant within the required precision.

Terminal velocity can also change during a fall. Air density varies with altitude, and an object may change orientation or deploy a parachute. If conditions change, the forces become unbalanced and the object accelerates or decelerates toward a new terminal velocity.

Terminal velocity is not possible in a vacuum

In a vacuum, there is no surrounding fluid and therefore no drag. If gravity is the only significant force, the object continues accelerating rather than settling at a terminal speed.

Near Earth's surface and over a limited distance, the acceleration is modeled as approximately constant at $g$ . For extremely long falls, gravitational field strength itself changes with distance, so the simple constant- $g$ model is no longer sufficient, but this is not terminal velocity because there is still no drag balance.

This also explains why a feather and a hammer fall together in a vacuum. Their very different behavior in air comes primarily from drag relative to weight, not because gravity fundamentally accelerates the feather less.

How terminal velocity is tested in IB Physics

The current DP Physics course, first assessed in 2025, organizes mechanics within Theme A, including A.1 Kinematics and A.2 Forces and momentum, as shown in the official IB Physics subject brief. Terminal-velocity reasoning connects motion graphs, fluid resistance, force balance, and Newton's laws.

The official IB specimen materials demonstrate the central condition used in assessment: at terminal velocity, the relevant resistive forces balance the force driving the motion. Depending on the situation, the force balance may include weight, drag, and buoyancy.

For a standard falling-object explanation, write a complete causal sequence:

The object accelerates downward because weight is initially greater than drag.
Its speed relative to the air increases.
Drag increases with speed.
The resultant downward force decreases.
By $F_{net}=ma$ , the acceleration decreases.
Drag eventually equals weight, so resultant force and acceleration become zero.
The object continues at constant terminal velocity.

For broader mechanics review, use the exam-focused IB Physics Kinematics Explained guide. It places terminal speed within the wider treatment of motion graphs, acceleration, projectiles, and model selection without replacing the focused explanation here.

Common mistakes to avoid

Saying gravity disappears: Gravity and weight continue acting at terminal velocity.
Saying the object stops: It stops accelerating, not moving.
Calling drag and weight a Newton's third-law pair: Both forces act on the falling object, so they are not an action-reaction pair.
Assuming acceleration is constant: When drag changes with speed, the resultant force and acceleration also change.
Using constant-acceleration equations across the whole fall: The SUVAT equations require constant acceleration and generally cannot model the complete approach to terminal speed.
Assuming every object has the same terminal velocity: Mass, area, shape, orientation, fluid density, and drag behavior all matter.
Applying quadratic drag automatically: Some low-speed viscous situations are better modeled by linear drag or Stokes' law.
Ignoring buoyancy in liquids: Buoyancy may form a significant part of the terminal force balance.

Targeted practice is more useful than memorizing a definition alone. RevisionDojo's A.2 Forces and Momentum notes can reinforce free-body diagrams and Newton's laws, while the A.2 Forces and Momentum Questionbank provides exam-style application.

Conclusion

Terminal velocity exists because drag is speed-dependent. A falling object initially accelerates under its weight, but increasing speed produces increasing drag, reducing the resultant force until drag balances weight. At that point, resultant force and acceleration are zero, so the object continues at constant velocity.

The terminal value differs between objects because weight, projected area, shape, orientation, and fluid properties determine the speed required for force balance. For IB exam preparation, combine this explanation with free-body diagrams, graph interpretation, and careful use of Newton's second law. RevisionDojo's IB Physics resources, topic Questionbank, flashcards, and Jojo AI can help you test whether you can apply the reasoning rather than only recall the definition.

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For IB Physics, the central chain of reasoning is:

speed increases → drag increases → resultant force decreases → acceleration decreases → drag balances weight → terminal velocity