Displacement (vector)

Classical mechanics
$\vec{F} = m\vec{a}$ Second law of motion
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Branches Applied Celestial Continuum Dynamics Kinematics Kinetics Statics Statistical
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Formulations Newton's laws of motion Analytical mechanics Lagrangian mechanics Hamiltonian mechanics Routhian mechanics Hamilton–Jacobi equation Appell's equation of motion Udwadia–Kalaba equation
Core topics Damping (ratio) Displacement Equations of motion Euler's laws of motion Fictitious force Friction Harmonic oscillator Inertial / Non-inertial reference frame Mechanics of planar particle motion Motion (linear) Newton's law of universal gravitation Newton's laws of motion Relative velocity Rigid body dynamics Euler's equations Simple harmonic motion Vibration
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Scientists Galileo Newton Kepler Horrocks Halley Euler d'Alembert Clairaut Lagrange Laplace Hamilton Poisson Daniel Bernoulli Johann Bernoulli Cauchy

Displacement versus distance traveled along a path

A displacement is a vector that is the shortest distance from the initial to the final position of a point P.^[1] It quantifies both the distance and direction of an imaginary motion along a straight line from the initial position to the final position of the point.

A displacement may be also described as a 'relative position': the final position of a point (R_f) relative to its initial position (R_i), and a displacement vector can be mathematically defined as the difference between the final and initial position vectors:

\boldsymbol{s}=\boldsymbol{R_f-R_i}=\Delta\boldsymbol{R}

In considering motions of objects over time the instantaneous velocity of the object is the rate of change of the displacement as a function of time. The velocity then is distinct from the instantaneous speed which is the time rate of change of the distance traveled along a specific path. The velocity may be equivalently defined as the time rate of change of the position vector. If one considers a moving initial position, or equivalently a moving origin (e.g. an initial position or origin which is fixed to a train wagon, which in turn moves with respect to its rail track), the velocity of P (e.g. a point representing the position of a passenger walking on the train) may be referred to as a relative velocity, as opposed to an absolute velocity, which is computed with respect to a point which is considered to be 'fixed in space' (such as, for instance, a point fixed on the floor of the train station).

For motion over a given interval of time, the displacement divided by the length of the time interval defines the average velocity. (Note that the average velocity, as a vector, differs from the average speed that is the ratio of the path length — a scalar — and the time interval.)

Rigid body

In dealing with the motion of a rigid body, the term displacement may also include the rotations of the body. In this case, the displacement of a particle of the body is called linear displacement (displacement along a line), while the rotation of the body is called angular displacement.

Derivatives

For a position vector s that is a function of time t, the derivatives can be computed with respect to t. These derivatives have common utility in the study of kinematics, control theory, vibration sensing and other sciences and engineering disciplines.

Velocity

\boldsymbol{v}=\frac{\text{d}\boldsymbol{s}}{\text{d}t}

(where ds is an infinitesimally small displacement)

Acceleration

\boldsymbol{a}=\frac{\text{d}\boldsymbol{v}}{\text{d}t}=\frac{\text{d}^2\boldsymbol{s}}{\text{d}t^2}

Jerk

\boldsymbol{j}=\frac{\text{d}\boldsymbol{a}}{\text{d}t}=\frac{\text{d}^2\boldsymbol{v}}{\text{d}t^2}=\frac{\text{d}^3\boldsymbol{s}}{\text{d}t^3}

These common names correspond to terminology used in basic kinematics.^[2] By extension, the higher order derivatives can be computed in a similar fashion. Study of these higher order derivatives can improve approximations of the original displacement function. Such higher-order terms are required in order to accurately represent the displacement function as a sum of an infinite series, enabling several analytical techniques in engineering and physics. The fourth order derivative is called jounce, the fifth crackle, and the sixth pop.

References

↑ Tom Henderson. "Describing Motion with Words". The Physics Classroom. Retrieved 2 January 2012.
↑ Stewart, James (2001). "§2.8 - The Derivative As A Function". Calculus (2nd ed.). Brooks/Cole. ISBN 0-534-37718-1.

Kinematics

← Integrate … Differentiate →

Absement Displacement (Distance) Velocity (Speed) Acceleration Jerk Jounce Crackle Pop

Classical mechanics SI units

Dimensions	—	L	L²	Dimensions	—	—	—
Linear/translational quantities				Angular/rotational quantities
T	time: $t$ s	absement: $A$ m s		T	time: $t$ s
—		distance: $d$ , position: $r$ , $s$ , $x$ , displacement m	area: $A$ m²	—		angle: $θ$ , angular displacement: $θ$ rad	solid angle: $Ω$ rad², sr
T⁻¹	frequency: $f$ s⁻¹, Hz	speed: $v$ , velocity: $v$ m s⁻¹	kinematic viscosity: $ν$ , specific angular momentum: $h$ m² s⁻¹	T⁻¹	frequency: $f$ s⁻¹, Hz	angular speed: $ω$ , angular velocity: $ω$ rad s⁻¹
T⁻²		acceleration: $a$ m s⁻²		T⁻²		angular acceleration: $α$ rad s⁻²
T⁻³		jerk: $j$ m s⁻³		T⁻³		angular jerk: $ζ$ rad s⁻³

M	mass: $m$ kg			ML²	moment of inertia: $I$ kg m²
MT⁻¹		momentum: $p$ , impulse: $J$ kg m s⁻¹, N s	action: $𝒮$ , actergy: $ℵ$ kg m² s⁻¹, J s	ML²T⁻¹		angular momentum: $L$ , angular impulse: $Δ L$ kg m² s⁻¹	action: $𝒮$ , actergy: $ℵ$ kg m² s⁻¹, J s
MT⁻²		force: $F$ , weight: $F g$ kg m s⁻², N	energy: $E$ , work: $W$ kg m² s⁻², J	ML²T⁻²		torque: $τ$ , moment: $M$ kg m² s⁻², N m	energy: $E$ , work: $W$ kg m² s⁻², J
MT⁻³		yank: $Y$ kg m s⁻³, N s⁻¹	power: $P$ kg m² s⁻³, W	ML²T⁻³		rotatum: $P$ kg m² s⁻³, N m s⁻¹	power: $P$ kg m²s⁻³, W

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Displacement (vector)

Rigid body

Derivatives

See also

References