Source code for sympy.physics.mechanics.particle

from __future__ import print_function, division

__all__ = ['Particle']

from sympy import sympify
from sympy.physics.mechanics.point import Point

[docs]class Particle(object):
"""A particle.

Particles have a non-zero mass and lack spatial extension; they take up no
space.

Values need to be supplied on initialization, but can be changed later.

Parameters
==========
name : str
Name of particle
point : Point
A physics/mechanics Point which represents the position, velocity, and
acceleration of this Particle
mass : sympifyable
A SymPy expression representing the Particle's mass

Examples
========

>>> from sympy.physics.mechanics import Particle, Point
>>> from sympy import Symbol
>>> po = Point('po')
>>> m = Symbol('m')
>>> pa = Particle('pa', po, m)
>>> # Or you could change these later
>>> pa.mass = m
>>> pa.point = po

"""

def __init__(self, name, point, mass):
if not isinstance(name, str):
raise TypeError('Supply a valid name.')
self._name = name
self.set_mass(mass)
self.set_point(point)
self._pe = sympify(0)

def __str__(self):
return self._name

__repr__ = __str__

[docs]    def get_mass(self):
"""Mass of the particle."""
return self._mass

def set_mass(self, mass):
self._mass = sympify(mass)

mass = property(get_mass, set_mass)

[docs]    def get_point(self):
"""Point of the particle."""
return self._point

def set_point(self, p):
if not isinstance(p, Point):
raise TypeError("Particle point attribute must be a Point object.")
self._point = p

point = property(get_point, set_point)

[docs]    def linear_momentum(self, frame):
"""Linear momentum of the particle.

The linear momentum L, of a particle P, with respect to frame N is
given by

L = m * v

where m is the mass of the particle, and v is the velocity of the
particle in the frame N.

Parameters
==========

frame : ReferenceFrame
The frame in which linear momentum is desired.

Examples
========

>>> from sympy.physics.mechanics import Particle, Point, ReferenceFrame
>>> from sympy.physics.mechanics import dynamicsymbols
>>> m, v = dynamicsymbols('m v')
>>> N = ReferenceFrame('N')
>>> P = Point('P')
>>> A = Particle('A', P, m)
>>> P.set_vel(N, v * N.x)
>>> A.linear_momentum(N)
m*v*N.x

"""

return self.mass * self.point.vel(frame)

[docs]    def angular_momentum(self, point, frame):
"""Angular momentum of the particle about the point.

The angular momentum H, about some point O of a particle, P, is given
by:

H = r x m * v

where r is the position vector from point O to the particle P, m is
the mass of the particle, and v is the velocity of the particle in
the inertial frame, N.

Parameters
==========

point : Point
The point about which angular momentum of the particle is desired.

frame : ReferenceFrame
The frame in which angular momentum is desired.

Examples
========

>>> from sympy.physics.mechanics import Particle, Point, ReferenceFrame
>>> from sympy.physics.mechanics import dynamicsymbols
>>> m, v, r = dynamicsymbols('m v r')
>>> N = ReferenceFrame('N')
>>> O = Point('O')
>>> A = O.locatenew('A', r * N.x)
>>> P = Particle('P', A, m)
>>> P.point.set_vel(N, v * N.y)
>>> P.angular_momentum(O, N)
m*r*v*N.z

"""

return self.point.pos_from(point) ^ (self.mass * self.point.vel(frame))

[docs]    def kinetic_energy(self, frame):
"""Kinetic energy of the particle

The kinetic energy, T, of a particle, P, is given by

'T = 1/2 m v^2'

where m is the mass of particle P, and v is the velocity of the
particle in the supplied ReferenceFrame.

Parameters
==========

frame : ReferenceFrame
The Particle's velocity is typically defined with respect to
an inertial frame but any relevant frame in which the velocity is
known can be supplied.

Examples
========

>>> from sympy.physics.mechanics import Particle, Point, ReferenceFrame
>>> from sympy import symbols
>>> m, v, r = symbols('m v r')
>>> N = ReferenceFrame('N')
>>> O = Point('O')
>>> P = Particle('P', O, m)
>>> P.point.set_vel(N, v * N.y)
>>> P.kinetic_energy(N)
m*v**2/2

"""

return (self.mass / sympify(2) * self.point.vel(frame) &
self.point.vel(frame))

[docs]    def set_potential_energy(self, scalar):
"""Used to set the potential energy of the Particle.

Parameters
==========

scalar : Sympifyable
The potential energy (a scalar) of the Particle.

Examples
========

>>> from sympy.physics.mechanics import Particle, Point
>>> from sympy import symbols
>>> m, g, h = symbols('m g h')
>>> O = Point('O')
>>> P = Particle('P', O, m)
>>> P.set_potential_energy(m * g * h)

"""

self._pe = sympify(scalar)

@property
[docs]    def potential_energy(self):
"""The potential energy of the Particle.

Examples
========

>>> from sympy.physics.mechanics import Particle, Point
>>> from sympy import symbols
>>> m, g, h = symbols('m g h')
>>> O = Point('O')
>>> P = Particle('P', O, m)
>>> P.set_potential_energy(m * g * h)
>>> P.potential_energy
g*h*m

"""

return self._pe