linear momenta

Rigid body dynamics

In physics, rigid body dynamics is the study of the motion of rigid bodies. Unlike particles, which move only in three degrees of freedom (translation in three directions), rigid bodies occupy space and have geometrical properties, such as a center of mass, moments of inertia, etc., that characterize motion in six degrees of freedom (translation in three directions plus rotation in three directions). Rigid bodies are also characterized as being non-deformable, as opposed to deformable bodies. As such, rigid body dynamics is used heavily in analyses and computer simulations of physical systems and machinery where rotational motion is important, but material deformation does not have a major effect on the motion of the system.

Rigid body linear momentum

Newton's Second Law states that the rate of change of the linear momentum of a particle with constant mass is equal to the sum of all external forces acting on the particle:

frac{mathrm{d}(m mathbf{v})}{mathrm{d}t}=sum_{i=1}^N mathbf{f}_i

where m is the particle's mass, v is the particle's velocity, their product mv is the linear momentum, and fi is one of the N number of forces acting on the particle.

Because the mass is constant, this is equivalent to

m frac{mathrm{d}mathbf{v}}{mathrm{d}t}=sum_{i=1}^N mathbf{f}_i.

To generalize, assume a body of finite mass and size is composed of such particles, each with infinitesimal mass dm. Each particle a position vector r. There exist internal forces, acting between any two particles, and external forces, acting only on the outside of the mass. Since velocity v is the derivative of position r, the derivative of velocity dv/dt is the second derivative of position d2r/dt2, and the linear momentum equation of any given particle is

mathrm{d}m frac{mathrm{d}^2mathbf{r}}{mathrm{d}t^2}= sum_{i=1}^M mathbf{f}_{i,text{internal}} + sum_{j=1}^N mathbf{f}_{j,mathrm{external}}.

When the linear momentum equations for all particles are added together, the internal forces sum to zero according to Newton's third law, which states that any such force has opposite magnitudes on the two particles. By accounting for all particles, the left side becomes an integral over the entire body, and the second derivative operator can be moved out of the integral, so

frac{mathrm{d}^2}{mathrm{d}t^2} int mathbf{r}, mathrm{d}m = sum_{j=1}^N mathbf{f}_{j,mathrm{external}}.

Let M be the total mass, which is constant, so the left side can be multiplied and divided by M, so

M frac{mathrm{d}^2}{mathrm{d}t^2}!left(frac{int mathbf{r}, mathrm{d}m}{M}right) = sum_{j=1}^N mathbf{f}_{j,mathrm{external}}.

The expression frac{int mathbf{r}, mathrm{d}m}{M} is the formula for the position of the center of mass. Denoting this by rcm, the equation reduces to

M frac{mathrm{d}^2 mathbf{r}_mathrm{cm}}{mathrm{d}t^2} = sum_{j=1}^N mathbf{f}_{j,mathrm{external}}.

Thus, linear momentum equations can be extended to rigid bodies by denoting that they describe the motion of the center of mass of the body.

Rigid body angular momentum

The most general equation for rotation of a rigid body in three dimensions about an arbitrary origin O with axes x, y, z is

M b_{G/O} times frac{mathrm{d}^2 R_O}{mathrm{d}t^2} + frac{mathrm{d}(mathbf{I}boldsymbol{omega})}{mathrm{d}t} = sum_{j=1}^N tau_{O,j}

where the moment of inertia tensor, mathbf{I}, is given by

mathbf{I} = begin{pmatrix}
I_{xx} & I_{xy} & I_{xz} I_{yx} & I_{yy} & I_{yz} I_{zx} & I_{zy} & I_{zz} end{pmatrix}

mathbf{I} =
begin{pmatrix} int (y^2+z^2), mathrm{d}m & -int xy, mathrm{d}m & -int xz, mathrm{d}m -int xy, mathrm{d}m & int (x^2+z^2), mathrm{d}m & -int yz, mathrm{d}m -int xz, mathrm{d}m & -int yz, mathrm{d}m & int (x^2+y^2), mathrm{d}m end{pmatrix}

and the angular velocity, boldsymbol{omega}, is given by

quad boldsymbol{omega} = omega_x mathbf{hat{i}} + omega_y mathbf{hat{j}} + omega_z mathbf{hat{k}}

where scriptstyle{(mathbf{hat{i}}, mathbf{hat{j}}, mathbf{hat{k}})} is a set of mutually perpendicular unit vectors fixed in a reference frame.

Moving any rigid body is equivalent to moving a Poinsot's ellipsoid.

Angular momentum and torque

Similarly, the angular momentum mathbf{L} for a system of particles with linear momenta p_{i} and distances r_{i} from the rotation axis is defined

mathbf{L} = sum_{i=1}^{N} mathbf{r}_{i} times mathbf{p}_{i} = sum_{i=1}^{N} m_{i} mathbf{r}_{i} times mathbf{v}_{i}

For a rigid body rotating with angular velocity omega about the rotation axis mathbf{hat{n}} (a unit vector), the velocity vector mathbf{v}_{i} may be written as a vector cross product

mathbf{v}_{i} = omega mathbf{hat{n}} times mathbf{r}_{i} stackrel{mathrm{def}}{=} boldsymbolomega times mathbf{r}_{i}


angular velocity vector boldsymbolomega stackrel{mathrm{def}}{=} omega mathbf{hat{n}}
mathbf{r}_{i} is the shortest vector from the rotation axis to the point mass.

Substituting the formula for mathbf{v}_{i} into the definition of mathbf{L} yields

mathbf{L} = sum_{i=1}^{N} m_{i} mathbf{r}_{i} times (boldsymbolomega times mathbf{r}_{i}) = boldsymbolomega sum_{i=1}^{N} m_{i} r_{i}^{2} = I omega mathbf{hat{n}}

where we have introduced the special case that the position vectors of all particles are perpendicular to the rotation axis (e.g., a flywheel): boldsymbolomega cdot mathbf{r}_{i} = 0.

The torque mathbf{N} is defined as the rate of change of the angular momentum mathbf{L}

mathbf{N} stackrel{mathrm{def}}{=} frac{dmathbf{L}}{dt}

If I is constant (because the inertia tensor is the identity, because we work in the intrinsecal frame, or because the torque is driving the rotation around the same axis mathbf{hat{n}} so that mathrm{I} is not changing) then we may write

mathbf{N} stackrel{mathrm{def}}{=} I frac{domega}{dt}mathbf{hat{n}} = I alpha mathbf{hat{n}}


alpha is called the angular acceleration (or rotational acceleration) about the rotation axis mathbf{hat{n}}.

Notice that if I is not constant in the external reference frame (ie. the three main axes of the body are different) then we cannot take the I outside the derivate. In this cases we can have torque-free precession.


Computer physics engines use rigid body dynamics to increase interactivity and realism in video games.

See also



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