Relative velocity

Relative velocity is the velocity of an object with respect to another object or reference frame, determined by vector addition of the object's velocity relative to an intermediate frame and the velocity of that intermediate frame relative to the observer's frame.^[1] In classical mechanics, this addition follows Galilean principles, where velocities are simply superimposed as vectors, applicable for speeds much less than the speed of light.^[2] For instance, if a person walks eastward at 2 m/s inside a train moving eastward at 10 m/s relative to Earth, the person's velocity relative to Earth is 12 m/s eastward.^[2] This concept is fundamental in kinematics for analyzing motion in multiple reference frames, such as a boat crossing a river with a current or an airplane navigating through wind.^[1] In one dimension, relative velocity is the algebraic sum of component velocities; in two dimensions, it requires resolving vectors into components and adding them separately along each axis.^[3] An example is a boat moving at 0.75 m/s perpendicular to a 1.20 m/s river current, resulting in a net velocity of 1.41 m/s at an angle of approximately 32° relative to the shore.^[3] These calculations highlight how relative motion affects observed paths, appearing straight in one frame but curved in another.^[3] At relativistic speeds, classical addition fails because it would allow velocities exceeding the speed of light (c = 3.00 × 10⁸ m/s), violating special relativity.^[4] Instead, the relativistic velocity addition formula is used: u = \frac{v + u'}{1 + \frac{v u'}{c^2}}, where v is the velocity of the moving frame relative to the observer, and u' is the object's velocity relative to the moving frame.^[4] For example, a spaceship traveling at 0.500c relative to Earth launches a probe at 0.750c in the same direction, yielding a probe velocity of 0.909c relative to Earth, less than the classical sum of 1.25c.^[4] This ensures the speed of light remains invariant across inertial frames, a cornerstone of Einstein's theory.^[4]

Basic Concepts

Definition and Notation

Relative velocity is defined as the velocity of one object with respect to another, specifically the time rate of change of the relative position vector between them.^[5] In vector terms, if two objects A and B have position vectors \vec{r}_A and \vec{r}_B relative to a common origin, the relative position is \vec{r}_{A/B} = \vec{r}_A - \vec{r}_B, and the relative velocity is \vec{v}_{A/B} = \frac{d}{dt} \vec{r}_{A/B}.^[5] This concept emphasizes that velocity is not absolute but depends on the observer's frame of reference.^[6] Standard notation distinguishes relative velocity from absolute velocities measured in a fixed frame. The velocity of object A relative to B is denoted \vec{v}_{A/B}, where absolute velocities are \vec{v}_A and \vec{v}_B. In inertial frames under classical mechanics, this satisfies \vec{v}_{A/B} = \vec{v}_A - \vec{v}_B.^[7] This subscript convention, with the numerator indicating the object and the denominator the reference, facilitates clear expression of motion differences.^[5] The idea of relative velocity originated in the early 17th century with Galileo Galilei's qualitative discussions of motion observed from different viewpoints, as explored in his 1632 work Dialogue Concerning the Two Chief World Systems.^[8] Galileo illustrated this through thought experiments, such as observing events on a moving ship indistinguishable from those on a stationary one, without employing modern equations.^[8] In everyday scenarios, relative velocity manifests as the scalar difference in speeds; for instance, if two cars travel in the same direction on a highway at 100 km/h and 80 km/h, their relative speed is 20 km/h.^[1]

Reference Frames and Observers

Inertial reference frames are non-accelerating coordinate systems in which Newton's laws of motion hold without fictitious forces, such that objects subject to no net external force either remain at rest or continue in uniform rectilinear motion.^[9] In these frames, the relative velocity between two objects, defined as the vector difference in their velocities, has a direction that remains unchanged across different inertial frames for speeds much less than the speed of light, as the transformation between frames involves only a constant velocity shift.^[10] This invariance underscores that relative velocity is not an absolute property but one that depends on the chosen frame for measurement.^[10] The dependence on observers arises because different inertial frames yield different absolute velocity measurements for the same object. For instance, an observer in frame S measures velocities \vec{v}_A and \vec{v}_B for two objects A and B, computing their relative velocity as \vec{v}_{A/B} = \vec{v}_A - \vec{v}_B. An observer in frame S', moving at constant velocity \vec{u} relative to S, measures \vec{v}_A' = \vec{v}_A - \vec{u} and \vec{v}_B' = \vec{v}_B - \vec{u}, resulting in the same relative velocity \vec{v}_{A/B}' = \vec{v}_A' - \vec{v}_B' = \vec{v}_{A/B}.^[10] Thus, relative velocity is inherently relational, characterizing the motion of one object with respect to another rather than with respect to an absolute space.^[9] The Galilean principle of relativity states that the laws of classical mechanics are identical in all inertial frames, meaning no experiment can distinguish one inertial frame from another through mechanical observations alone.^[11] Under uniform translation between frames, relative velocities are preserved, ensuring the equivalence of descriptions across observers.^[10] A classic illustration is Galileo's thought experiment with a ship: a passenger below deck, shielded from external cues, cannot detect the ship's uniform motion relative to the shore, perceiving dropped balls or tossed objects as moving straight down or horizontally as if at rest; an observer on shore sees the same internal motions plus the ship's overall translation, yet the relative velocities within the ship remain unchanged, demonstrating the absence of absolute rest.^[9]

Non-Relativistic Mechanics

One-Dimensional Case

In non-relativistic mechanics, the one-dimensional case of relative velocity applies to objects moving collinearly along a straight line, where velocities are treated as signed scalars to account for direction. The relative velocity v_{\text{rel}} of object 1 with respect to object 2 is defined as the difference in their velocities, v_{\text{rel}} = v_1 - v_2, measured in a common inertial reference frame.^[12] This scalar quantity determines how the separation between the objects changes over time and remains invariant across inertial frames under Galilean transformations.^[13] The formula arises directly from the definition of velocity as the time derivative of position. Consider the relative position x_{\text{rel}} = x_1 - x_2, where x_1 and x_2 are the positions of the two objects along the line. Differentiating with respect to time yields

v_{\text{rel}} = \frac{dx_{\text{rel}}}{dt} = \frac{dx_1}{dt} - \frac{dx_2}{dt} = v_1 - v_2.

This derivation assumes smooth, differentiable motion and holds for any inertial observer, as the relative velocity is frame-independent in classical mechanics.^[14]^[12] Practical scenarios illustrate the concept. For two objects approaching each other along the line, such as vehicles moving in opposite directions with v_1 = +10 m/s and v_2 = -10 m/s, the relative velocity is v_{\text{rel}} = 20 m/s, and the closing speed—the rate at which the distance decreases—is the magnitude |v_{\text{rel}}| = 20 m/s.^[13] In an overtaking situation, where both move in the same direction but object 1 is faster (e.g., v_1 = 20 m/s and v_2 = 15 m/s), v_{\text{rel}} = 5 m/s indicates object 1 gaining on object 2 at that rate.^[13] Head-on collision scenarios, common in elastic or inelastic impacts, rely on this subtraction; for instance, in a one-dimensional elastic collision between two masses, the relative velocity reverses sign post-collision (v_{\text{rel},f} = -v_{\text{rel},i}), conserving momentum and kinetic energy.^[12] This treatment assumes constant velocities, ignoring acceleration, and is valid only for speeds much less than the speed of light (v \ll c), where relativistic effects like velocity addition are negligible.^[14]^[12] At higher speeds, the classical subtraction fails, as demonstrated by cases where adding velocities near c exceeds c in the non-relativistic formula but does not in relativity.^[14]

Multi-Dimensional Case

In non-relativistic mechanics, the relative velocity between two objects A and B in multiple dimensions is represented as a vector difference, \vec{v}_{A/B} = \vec{v}_A - \vec{v}_B, where \vec{v}_A and \vec{v}_B are the velocity vectors of A and B with respect to a common inertial reference frame.^[15]^[16] This formulation extends the one-dimensional case, which treats velocities as scalars along a single axis, to account for motion in two or three dimensions using vector algebra. The components of the relative velocity vector are obtained by subtracting the corresponding components of the individual velocities: v_{A/B,x} = v_{A,x} - v_{B,x}, v_{A/B,y} = v_{A,y} - v_{B,y}, and v_{A/B,z} = v_{A,z} - v_{B,z} in three dimensions.^[15] The magnitude, or relative speed, is the scalar value |\vec{v}_{A/B}| = |\vec{v}_A - \vec{v}_B|, calculated using the Euclidean norm, while the direction is determined by the orientation of the resulting vector difference.^[16] Geometrically, this vector subtraction can be visualized using the parallelogram rule, where the relative velocity vector forms the diagonal of a parallelogram constructed from \vec{v}_A and -\vec{v}_B.^[16] A practical example in two dimensions involves two ships crossing paths: suppose ship A moves eastward at 10 m/s and ship B moves northward at 15 m/s, both relative to the water. The relative velocity of A with respect to B is then \vec{v}_{A/B} = (10 \, \hat{i} - 15 \, \hat{j}) m/s, with a magnitude of \sqrt{10^2 + 15^2} = \sqrt{325} \approx 18.0 m/s directed at an angle of \tan^{-1}(15/10) \approx 56.3^\circ south of east.^[16] In three dimensions, consider an aircraft flying with an airspeed of 200 m/s due north while encountering wind with components of 30 m/s east and 20 m/s upward; the ground-relative velocity components must account for all three directions to determine the actual path.^[15] To analyze complex motions, the relative velocity vector can be decomposed into components parallel and perpendicular to a chosen reference direction, such as the line of sight between objects or a navigational heading.^[16] For instance, in the aircraft example, the wind's parallel component affects speed along the intended path, while the perpendicular component causes deviation, requiring adjustments like heading corrections to maintain course.^[15] This decomposition aids in applications like collision avoidance or trajectory prediction by isolating effects in each dimension.

Galilean Transformations

In classical mechanics, the concept of relative velocity between inertial frames is encapsulated by the Galilean transformations, which describe how position and time coordinates change between two frames moving at a constant relative velocity. These transformations were implicitly motivated by Galileo's thought experiment in his 1632 work Dialogue Concerning the Two Chief World Systems, where he imagined a ship sailing smoothly and uniformly across a calm sea. An observer below deck, shielded from external cues, could perform any mechanical experiment—such as dropping a ball or observing a fly's flight—and obtain identical results to those on a stationary ship, demonstrating that uniform motion is undetectable and that physical laws are the same in all inertial frames.^[17]^[18] Consider two inertial reference frames, S and S', where S' moves with constant velocity \vec{u} relative to S along the x-axis, and the origins coincide at t = 0. The Galilean transformation equations for position and time are:

x' = x - u t, \quad y' = y, \quad z' = z, \quad t' = t

These assume absolute time, independent of the frame. To derive the velocity transformation, differentiate the position equations with respect to time, treating the particle's position as a function of time. For the x-component in one dimension, the velocity in S is v_x = dx/dt, so in S' it follows that v_x' = dx'/dt' = dx/dt - u = v_x - u, since dt' = dt. This relation extends to the vector form for arbitrary directions: \vec{v}' = \vec{v} - \vec{u}, where \vec{v} is the velocity measured in S and \vec{v}' in S'.^[14]^[10] A key consequence is the invariance of relative velocity in the non-relativistic regime. If two objects have velocities \vec{v}_1 and \vec{v}_2 in frame S, their relative velocity is \vec{v}_{rel} = \vec{v}_1 - \vec{v}_2. In frame S', these become \vec{v}_1' = \vec{v}_1 - \vec{u} and \vec{v}_2' = \vec{v}_2 - \vec{u}, yielding \vec{v}_{rel}' = \vec{v}_1' - \vec{v}_2' = (\vec{v}_1 - \vec{u}) - (\vec{v}_2 - \vec{u}) = \vec{v}_1 - \vec{v}_2 = \vec{v}_{rel}. Thus, all inertial observers agree on the relative velocity between the objects, underscoring the equivalence of frames under constant relative motion. This additive velocity rule holds for speeds much less than the speed of light, forming the basis for classical descriptions of relative motion.^[10]^[14]

Relativistic Mechanics

Lorentz Transformations for Velocities

In special relativity, the Lorentz transformations describe how coordinates and times transform between two inertial frames S and S', where S' moves at constant velocity v along the positive x-axis relative to S.^[19] The relevant transformations for position and time are

x' = \gamma (x - v t), \quad y' = y, \quad z' = z, \quad t' = \gamma \left( t - \frac{v x}{c^2} \right),

where \gamma = \frac{1}{\sqrt{1 - \frac{v^2}{c^2}}} and c is the speed of light.^[19] These equations account for the relativity of simultaneity and the finite speed of light, replacing the absolute time of classical mechanics.^[20] To derive the transformation of velocities, consider an object with velocity components \mathbf{u} = (u_x, u_y, u_z) in frame S, where u_x = \frac{dx}{dt}, u_y = \frac{dy}{dt}, and u_z = \frac{dz}{dt}. The components in S' are \mathbf{u}' = (u_x', u_y', u_z'), defined as u_x' = \frac{dx'}{dt'}, and similarly for the others.^[20] Differentiating the Lorentz transformations infinitesimally yields dx' = \gamma (dx - v \, dt), dy' = dy, dz' = dz, and dt' = \gamma \left( dt - \frac{v \, dx}{c^2} \right).^[19] For the parallel component, substituting the differentials gives

u_x' = \frac{dx'}{dt'} = \frac{\gamma (dx - v \, dt)}{\gamma \left( dt - \frac{v \, dx}{c^2} \right)} = \frac{u_x - v}{1 - \frac{u_x v}{c^2}}.

This formula shows that velocities do not add vectorially as in classical mechanics; the denominator arises from the non-additivity of times due to the relativity of simultaneity and time dilation effects.^[20] For the perpendicular components,

u_y' = \frac{dy'}{dt'} = \frac{dy}{\gamma \left( dt - \frac{v \, dx}{c^2} \right)} = \frac{u_y}{\gamma \left( 1 - \frac{u_x v}{c^2} \right)},

and similarly for u_z', reflecting the influence of the frame motion on the measurement of transverse velocities through the dilated time interval.^[20] In the low-speed limit where v \ll c and u_x \ll c, the Lorentz factor \gamma \approx 1 and the denominator terms approach unity, reducing the formulas to the Galilean velocity addition u_x' \approx u_x - v and u_y' \approx u_y.^[19] This ensures consistency with non-relativistic mechanics while preventing velocities from exceeding c.^[21]

Collinear Velocities

In special relativity, the relative velocity between two objects moving collinearly—along the same straight line—must account for the invariance of the speed of light, leading to a non-intuitive addition formula derived from the Lorentz transformations.^[22] For velocities in the same direction, consider a frame S' moving at velocity v relative to S, with an object moving at u' in S'; the object's velocity w in S is given by

w = \frac{u' + v}{1 + \frac{u' v}{c^2}},

where c is the speed of light.^[22] For head-on approach (opposite directions), if two objects have speeds u and v relative to a common inertial frame, the relative velocity w as measured in one of those frames is

w = \frac{u + v}{1 + \frac{u v}{c^2}}.

^[23] This relativistic composition ensures that no relative speed exceeds c, a fundamental postulate preventing superluminal motion even when individual speeds approach c.^[22] A key property of these formulas is the strict adherence to the speed limit: the magnitude |w| is always less than c, regardless of how close u and v are to c. For instance, if a spaceship travels at $0.9c relative to Earth and launches a probe at $0.9c in the same direction relative to the spaceship, the probe's velocity relative to Earth is approximately $0.994c. For opposite directions, if two objects approach each other at $0.9c each relative to a midway rest frame, the relative velocity as measured by one is also approximately $0.994c.^[24] These cases highlight how relativity treats both co-directional boosts and approaching motions differently from Newtonian expectations, with the formula ensuring the light-speed barrier.^[24] This velocity addition is crucial in high-energy physics, such as particle accelerators, where successive boosts must be composed relativistically to accurately predict collision energies; for example, in the Large Hadron Collider, protons accelerated to near-c speeds require this formula to model their relative motions without violating causality.^[25] Similarly, in astrophysical scenarios like two spaceships approaching at high speeds, an observer on one would measure the other's approach at less than $1.8c, preserving the light-speed barrier.^[26] To illustrate the deviation from classical mechanics, consider the case where u = v = 0.5c:

Scenario	Classical Relative Velocity	Relativistic Relative Velocity
Velocity addition (same direction)	$1.0c	$0.8c
Head-on (opposite directions)	$1.0c	$0.8c

These values underscore how relativistic effects become significant even at half the speed of light, with the formulas derived directly from Lorentz transformations as detailed in prior sections.^[4]

Non-Collinear Velocities

In special relativity, the composition of velocities becomes particularly intricate when the relative motion between frames includes components perpendicular to the direction of the frame's velocity. Consider a reference frame S' moving with velocity \mathbf{v} along the x-axis relative to frame S, and an object with velocity \mathbf{u}' in S', decomposed into parallel (u'_\parallel) and perpendicular (u'_\perp) components with respect to \mathbf{v}. The velocity \mathbf{w} of the object in S has components given by

w_\parallel = \frac{u'_\parallel + v}{1 + \frac{u'_\parallel v}{c^2}}, \quad w_\perp = \frac{u'_\perp}{\gamma_v \left(1 + \frac{u'_\parallel v}{c^2}\right)},

where \gamma_v = \frac{1}{\sqrt{1 - \frac{v^2}{c^2}}} is the Lorentz factor for the frame velocity v, and c is the speed of light.^[27] This formulation arises from applying the Lorentz transformations to the differentials of position and time, ensuring consistency with the invariance of c. The perpendicular component w_\perp is notably affected by the factor \gamma_v > 1, which reduces its magnitude compared to the classical expectation of simple vector addition. This reduction stems from time dilation in the moving frame S': the time interval dt in S is longer than dt' in S' by \gamma_v, while perpendicular displacements remain unchanged (dy = dy'), leading to a slower perceived perpendicular velocity in S.^[28] Despite this contraction, the total speed |\mathbf{w}| remains less than c, preserving the relativistic speed limit; for instance, if u'_\parallel = 0, then w_\perp = u'_\perp / \gamma_v < u'_\perp, and as v \to c, w_\perp \to 0.^[27]^[29] In the limit where u'_\perp = 0, the formulas reduce to the collinear case. A classic illustration of these effects is the aberration of light, where light emitted perpendicular to the direction of motion in one frame appears tilted in the other. For light (u' = c) emitted at 90° in S', the angle \theta in S satisfies \cos \theta = \beta (with \beta = v/c), derived directly from the perpendicular velocity transformation, demonstrating how the direction contracts forward due to the combined influence of length contraction and time dilation.^[29]^[30] Another example arises in muon decay experiments with perpendicular magnetic fields, where the observed decay paths of muons exhibit contraction in the transverse direction relative to the lab frame's boost, consistent with the reduced w_\perp and highlighting the non-Euclidean nature of relativistic kinematics.^[28] To visualize this non-intuitive addition, velocity space diagrams represent velocities as points on a hyperbolic geometry within the unit disk (for v < [c](/page/Speed_of_light)), where the composition corresponds to Möbius transformations rather than Euclidean vector sums, emphasizing the curvature induced by relativity.^[27]

Arbitrary Directions

In relativistic mechanics, the velocity addition formula for arbitrary directions accounts for the fact that boosts in special relativity are direction-dependent, requiring a decomposition of the velocity vector into components parallel and perpendicular to the relative motion between frames. Consider two objects with velocities \vec{u} and \vec{v} measured in the same inertial frame; the relative velocity \vec{w} of the first with respect to the second is obtained by applying the velocity addition rule to \vec{u} \oplus (-\vec{v}). This yields

\vec{w} = \frac{ (\vec{u} - \vec{v})_\parallel + \frac{(\vec{u} - \vec{v})_\perp}{\gamma_v} }{1 - \frac{\vec{u} \cdot \vec{v}}{c^2} },

where (\vec{u} - \vec{v})_\parallel and (\vec{u} - \vec{v})_\perp are the components of \vec{u} - \vec{v} parallel and perpendicular to \vec{v}, respectively, \gamma_v = (1 - v^2/c^2)^{-1/2} is the Lorentz factor for \vec{v}, and c is the speed of light.^[31] This decomposition ensures the formula applies to any angle between \vec{u} and \vec{v}, reducing to the collinear case when the perpendicular component vanishes and to the perpendicular case when the velocities are orthogonal.^[27] To express this in component form, one typically aligns the coordinate system such that \vec{v} lies along the x-axis, with v_x = v and v_y = v_z = 0; the velocity \vec{u} = (u_x, u_y, u_z) in the lab frame then transforms to \vec{w} = (w_x, w_y, w_z) in the rest frame of the object moving at \vec{v} via the Lorentz boost components:

w_x = \frac{u_x - v}{1 - \frac{u_x v}{c^2}}, \quad w_y = \frac{u_y}{\gamma_v \left(1 - \frac{u_x v}{c^2}\right)}, \quad w_z = \frac{u_z}{\gamma_v \left(1 - \frac{u_x v}{c^2}\right)}.

For a general direction of \vec{v}, the full Lorentz boost matrix in arbitrary orientation is applied to the four-velocity, yielding the three-velocity components after projection; this matrix involves rotations to align with \vec{v} followed by the standard boost.^[32]^[31] This general formula exhibits key properties that uphold the principles of special relativity. It guarantees that |\vec{w}| < c for any |\vec{u}| < c and |\vec{v}| < c, preventing superluminal speeds even when velocities are nearly aligned with c.^[27] Additionally, the addition corresponds to composing Lorentz boosts, which preserves the Minkowski norm of the four-velocity (U^\mu U_\mu = -c^2), ensuring consistency of proper time across frames.^[31] In particle physics, this formula is essential for composing multiple boosts in non-collinear directions, such as tracking particle trajectories in accelerators or analyzing multi-stage decays.^[33] A practical application arises in cosmic ray physics, where ultra-relativistic particles arrive from arbitrary directions due to galactic magnetic fields; the formula allows computation of observed arrival angles and energies by adding the Earth's velocity relative to the cosmic rest frame, which is crucial for anisotropy studies and source identification.^[33] Similarly, in satellite navigation systems like GPS, relativistic corrections incorporate velocity addition for non-collinear orbital motions to maintain positional accuracy within meters.^[27]