from sympy.core.backend import Symbol
from sympy.physics.mechanics import (RigidBody, Particle, ReferenceFrame,
inertia)
from sympy.physics.vector import Point, Vector
__all__ = ['Body']
[docs]class Body(RigidBody, Particle):
"""
Body is a common representation of either a RigidBody or a Particle SymPy
object depending on what is passed in during initialization. If a mass is
passed in and central_inertia is left as None, the Particle object is
created. Otherwise a RigidBody object will be created.
The attributes that Body possesses will be the same as a Particle instance
or a Rigid Body instance depending on which was created. Additional
attributes are listed below.
Attributes
==========
name : string
The body's name
masscenter : Point
The point which represents the center of mass of the rigid body
frame : ReferenceFrame
The reference frame which the body is fixed in
mass : Sympifyable
The body's mass
inertia : (Dyadic, Point)
The body's inertia around its center of mass. This attribute is specific
to the rigid body form of Body and is left undefined for the Particle
form
loads : iterable
This list contains information on the different loads acting on the
Body. Forces are listed as a (point, vector) tuple and torques are
listed as (reference frame, vector) tuples.
Parameters
==========
name : String
Defines the name of the body. It is used as the base for defining
body specific properties.
masscenter : Point, optional
A point that represents the center of mass of the body or particle.
If no point is given, a point is generated.
mass : Sympifyable, optional
A Sympifyable object which represents the mass of the body. If no
mass is passed, one is generated.
frame : ReferenceFrame, optional
The ReferenceFrame that represents the reference frame of the body.
If no frame is given, a frame is generated.
central_inertia : Dyadic, optional
Central inertia dyadic of the body. If none is passed while creating
RigidBody, a default inertia is generated.
Examples
========
Default behaviour. This results in the creation of a RigidBody object for
which the mass, mass center, frame and inertia attributes are given default
values. ::
>>> from sympy.physics.mechanics import Body
>>> body = Body('name_of_body')
This next example demonstrates the code required to specify all of the
values of the Body object. Note this will also create a RigidBody version of
the Body object. ::
>>> from sympy import Symbol
>>> from sympy.physics.mechanics import ReferenceFrame, Point, inertia
>>> from sympy.physics.mechanics import Body
>>> mass = Symbol('mass')
>>> masscenter = Point('masscenter')
>>> frame = ReferenceFrame('frame')
>>> ixx = Symbol('ixx')
>>> body_inertia = inertia(frame, ixx, 0, 0)
>>> body = Body('name_of_body', masscenter, mass, frame, body_inertia)
The minimal code required to create a Particle version of the Body object
involves simply passing in a name and a mass. ::
>>> from sympy import Symbol
>>> from sympy.physics.mechanics import Body
>>> mass = Symbol('mass')
>>> body = Body('name_of_body', mass=mass)
The Particle version of the Body object can also receive a masscenter point
and a reference frame, just not an inertia.
"""
def __init__(self, name, masscenter=None, mass=None, frame=None,
central_inertia=None):
self.name = name
self.loads = []
if frame is None:
frame = ReferenceFrame(name + '_frame')
if masscenter is None:
masscenter = Point(name + '_masscenter')
if central_inertia is None and mass is None:
ixx = Symbol(name + '_ixx')
iyy = Symbol(name + '_iyy')
izz = Symbol(name + '_izz')
izx = Symbol(name + '_izx')
ixy = Symbol(name + '_ixy')
iyz = Symbol(name + '_iyz')
_inertia = (inertia(frame, ixx, iyy, izz, ixy, iyz, izx),
masscenter)
else:
_inertia = (central_inertia, masscenter)
if mass is None:
_mass = Symbol(name + '_mass')
else:
_mass = mass
masscenter.set_vel(frame, 0)
# If user passes masscenter and mass then a particle is created
# otherwise a rigidbody. As a result a body may or may not have inertia.
if central_inertia is None and mass is not None:
self.frame = frame
self.masscenter = masscenter
Particle.__init__(self, name, masscenter, _mass)
else:
RigidBody.__init__(self, name, masscenter, frame, _mass, _inertia)
[docs] def apply_force(self, vec, point=None):
"""
Adds a force to a point (center of mass by default) on the body.
Parameters
==========
vec: Vector
Defines the force vector. Can be any vector w.r.t any frame or
combinations of frames.
point: Point, optional
Defines the point on which the force is applied. Default is the
Body's center of mass.
Example
=======
The first example applies a gravitational force in the x direction of
Body's frame to the body's center of mass. ::
>>> from sympy import Symbol
>>> from sympy.physics.mechanics import Body
>>> body = Body('body')
>>> g = Symbol('g')
>>> body.apply_force(body.mass * g * body.frame.x)
To apply force to any other point than center of mass, pass that point
as well. This example applies a gravitational force to a point a distance
l from the body's center of mass in the y direction. The force is again
applied in the x direction. ::
>>> from sympy import Symbol
>>> from sympy.physics.mechanics import Body
>>> body = Body('body')
>>> g = Symbol('g')
>>> l = Symbol('l')
>>> point = body.masscenter.locatenew('force_point', l *
... body.frame.y)
>>> body.apply_force(body.mass * g * body.frame.x, point)
"""
if not isinstance(point, Point):
if point is None:
point = self.masscenter # masscenter
else:
raise TypeError("A Point must be supplied to apply force to.")
if not isinstance(vec, Vector):
raise TypeError("A Vector must be supplied to apply force.")
self.loads.append((point, vec))
[docs] def apply_torque(self, vec):
"""
Adds a torque to the body.
Parameters
==========
vec: Vector
Defines the torque vector. Can be any vector w.r.t any frame or
combinations of frame.
Example
=======
This example adds a simple torque around the body's z axis. ::
>>> from sympy import Symbol
>>> from sympy.physics.mechanics import Body
>>> body = Body('body')
>>> T = Symbol('T')
>>> body.apply_torque(T * body.frame.z)
"""
if not isinstance(vec, Vector):
raise TypeError("A Vector must be supplied to add torque.")
self.loads.append((self.frame, vec))
