List Of Equations In Classical Mechanics

This page gives a summary of important equations in classical mechanics.

Nomenclature

a = acceleration (m/s²)
F = force (N = kg m/s²)
KE = kinetic energy (J = kg m²/s²)
m = mass (kg)
p = momentum (kg m/s)
s = position (m)
t = time (s)
v = velocity (m/s)
v0 = velocity at time t=0
W = work (J = kg m²/s²)
s(t) = position at time t
s0 = position at time t=0
runit = unit vector pointing from the origin in polar coordinates
θunit = unit vector pointing in the direction of increasing values of theta in polor coordinates
Note: All quantities in bold represent vectors.

Defining Equations

Center of Mass

In the discrete case:
\mathbf{s}_{\hbox{CM}} = {1 \over m_{\hbox{total}}} \sum_{i = 0}^{n} m_i \mathbf{s}_i
where n is the number of mass particles. Or in the continuous case:
\mathbf{s}_{\hbox{CM}} = {1 \over m_{\hbox{total}}} \int \rho(\mathbf{s}) dV
where ρ(s) is the scalar mass density as a function of the position vector.

Velocity

\mathbf{v}_{\mbox{average}} = {\Delta \mathbf{s} \over \Delta t}
\mathbf{v} = {d\mathbf{s} \over dt}

Acceleration

\mathbf{a}_{\mbox{average}} = \frac{\Delta\mathbf{v}}{\Delta t}
\mathbf{a} = \frac{d\mathbf{v}}{dt} = \frac{d^2\mathbf{s}}{dt^2}
  • Centripetal Acceleration
|\mathbf{a}_c | = \omega^2 R = v^2 / R
(R = radius of the circle, ω = v/R angular velocity)

Momentum

\mathbf{p} = m\mathbf{v}

Force

\sum \mathbf{F} = \frac{d\mathbf{p}}{dt} = \frac{d(m\mathbf{v})}{dt}
\sum \mathbf{F} = m\mathbf{a} \quad\   (Constant Mass)

Impulse

\mathbf{J} = \Delta \mathbf{p} = \int \mathbf{F} dt
\mathbf{J} = \mathbf{F} \Delta t \quad\
  if F is constant

Moment of Intertia

For a single axis of rotation:

Angular Momentum

|L| = mvr \quad\   if v is perpendicular to r
Vector form:
\mathbf{L} = \mathbf{r} \times \mathbf{p} = \mathbf{I}\, \omega
(Note: I can be treated like a vector if it is diagonalized first, but it is actually a 3×3 matrix) r is the radius vector

Torque

\sum \boldsymbol{\tau} = \frac{d\mathbf{L}}{dt}
\sum \boldsymbol{\tau} = \mathbf{r} \times \mathbf{F} \quad
if |r| and the sine of the angle between r and p remains constant.
\sum \boldsymbol{\tau} = \mathbf{I} \boldsymbol{\alpha}
This one is very limited, more added later. α = dω/dt

Precession

Energy

\Delta \mbox{ KE } = \int \mathbf{F}_{\mbox{net}} \cdot d\mathbf{s}
\mbox{KE } = \int \mathbf{v} \cdot d\mathbf{p} = \begin{matrix}\frac{1}{2}\end{matrix} mv^2 \quad\   if m is constant
\mbox{PE}_{\mbox{due to gravity}} = mgh \quad\
  (near the earth's surface)
g is the acceleration due to gravity, one of the physical constants at or near the earth's surface.

Central Force Motion

\frac{d^2}{d\theta^2}\left(\frac{1}{\mathbf{r}}\right) + \frac{1}{\mathbf{r}} = -\frac{\mu\mathbf{r}^2}{\mathbf{l}^2}\mathbf{F}(\mathbf{r})

Gravitational Force

\mathbf{F(r)} = -\frac{\mathbf{Gm_1}\mathbf{m_2}}{\mathbf{r^2}}
G is the gravitational constant, one of the physical constants

Useful derived equations

Position of an accelerating body

\mathbf{s}(t) = \begin{matrix}\frac{1}{2}\end{matrix} \mathbf{a} t^2 + \mathbf{v}_0 t + \mathbf{s}_0 \quad\   if a is constant.

Equation for velocity

v^2 =v_0^2 + 2\mathbf{a} \cdot \Delta\mathbf{s}

 

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