# Wrapping Geometry (Docstrings)#

Geometry objects for use by wrapping pathways.

A solid (infinite) cylindrical object.

Parameters:

point : Point

A point through which the cylinder’s axis passes.

axis : Vector

The axis along which the cylinder is aligned.

Explanation

A wrapping geometry that allows for circular arcs to be defined between pairs of points. These paths are always geodetic (the shortest possible) in the sense that they will be a straight line on the unwrapped cylinder’s surface. However, it is also possible for a direction to be specified, i.e. paths can be influenced such that they either wrap along the shortest side or the longest side of the cylinder. To define these directions, rotations are in the positive direction following the right-hand rule.

Examples

To create a WrappingCylinder instance, a Symbol denoting its radius, a Vector defining its axis, and a Point through which its axis passes are needed:

>>> from sympy import symbols
>>> from sympy.physics.mechanics import (Point, ReferenceFrame,
...     WrappingCylinder)
>>> N = ReferenceFrame('N')
>>> r = symbols('r')
>>> pO = Point('pO')
>>> ax = N.x


A cylinder with radius r, and axis parallel to N.x passing through pO can be instantiated with:

>>> WrappingCylinder(r, pO, ax)


WrappingSphere

Spherical geometry where the wrapping direction is always geodetic.

property axis#

Axis along which the cylinder is aligned.

geodesic_end_vectors(point_1, point_2)[source]#

The vectors parallel to the geodesic at the two end points.

Parameters:

point_1 : Point

The point from which the geodesic originates.

point_2 : Point

The point at which the geodesic terminates.

geodesic_length(point_1, point_2)[source]#

The shortest distance between two points on a geometry’s surface.

Parameters:

point_1 : Point

Point from which the geodesic length should be calculated.

point_2 : Point

Point to which the geodesic length should be calculated.

Explanation

The geodesic length, i.e. the shortest arc along the surface of a cylinder, connecting two points. It can be calculated using Pythagoras’ theorem. The first short side is the distance between the two points on the cylinder’s surface parallel to the cylinder’s axis. The second short side is the arc of a circle between the two points of the cylinder’s surface perpendicular to the cylinder’s axis. The resulting hypotenuse is the geodesic length.

Examples

A geodesic length can only be calculated between two points on the cylinder’s surface. Firstly, a WrappingCylinder instance must be created along with two points that will lie on its surface:

>>> from sympy import symbols, cos, sin
>>> from sympy.physics.mechanics import (Point, ReferenceFrame,
...     WrappingCylinder, dynamicsymbols)
>>> N = ReferenceFrame('N')
>>> r = symbols('r')
>>> pO = Point('pO')
>>> pO.set_vel(N, 0)
>>> cylinder = WrappingCylinder(r, pO, N.x)
>>> p1 = Point('p1')
>>> p2 = Point('p2')


Let’s assume that p1 is located at N.x + r*N.y relative to pO and that p2 is located at r*(cos(q)*N.y + sin(q)*N.z) relative to pO, where q(t) is a generalized coordinate specifying the angle rotated around the N.x axis according to the right-hand rule where N.y is zero. These positions can be set with:

>>> q = dynamicsymbols('q')
>>> p1.set_pos(pO, N.x + r*N.y)
>>> p1.pos_from(pO)
N.x + r*N.y
>>> p2.set_pos(pO, r*(cos(q)*N.y + sin(q)*N.z).normalize())
>>> p2.pos_from(pO).simplify()
r*cos(q(t))*N.y + r*sin(q(t))*N.z


The geodesic length, which is in this case a is the hypotenuse of a right triangle where the other two side lengths are 1 (parallel to the cylinder’s axis) and r*q(t) (parallel to the cylinder’s cross section), can be calculated using the geodesic_length method:

>>> cylinder.geodesic_length(p1, p2).simplify()
sqrt(r**2*q(t)**2 + 1)


If the geodesic_length method is passed an argument Point that doesn’t lie on the sphere’s surface then a ValueError is raised because it’s not possible to calculate a value in this case.

property point#

A point through which the cylinder’s axis passes.

point_on_surface(point)[source]#

Returns True if a point is on the cylinder’s surface.

Parameters:

point : Point

The point for which it’s to be ascertained if it’s on the cylinder’s surface or not. This point’s position relative to the cylinder’s axis must be a simple expression involving the radius of the sphere, otherwise this check will likely not work.

class sympy.physics.mechanics.wrapping_geometry.WrappingGeometryBase[source]#

Abstract base class for all geometry classes to inherit from.

Notes

Instances of this class cannot be directly instantiated by users. However, it can be used to created custom geometry types through subclassing.

abstract geodesic_end_vectors(point_1, point_2)[source]#

The vectors parallel to the geodesic at the two end points.

Parameters:

point_1 : Point

The point from which the geodesic originates.

point_2 : Point

The point at which the geodesic terminates.

abstract geodesic_length(point_1, point_2)[source]#

Returns the shortest distance between two points on a geometry’s surface.

Parameters:

point_1 : Point

The point from which the geodesic length should be calculated.

point_2 : Point

The point to which the geodesic length should be calculated.

abstract property point#

The point with which the geometry is associated.

abstract point_on_surface(point)[source]#

Returns True if a point is on the geometry’s surface.

Parameters:

point : Point

The point for which it’s to be ascertained if it’s on the geometry’s surface or not.

A solid spherical object.

Parameters:

Radius of the sphere. This symbol must represent a value that is positive and constant, i.e. it cannot be a dynamic symbol, nor can it be an expression.

point : Point

A point at which the sphere is centered.

Explanation

A wrapping geometry that allows for circular arcs to be defined between pairs of points. These paths are always geodetic (the shortest possible).

Examples

To create a WrappingSphere instance, a Symbol denoting its radius and Point at which its center will be located are needed:

>>> from sympy import symbols
>>> from sympy.physics.mechanics import Point, WrappingSphere
>>> r = symbols('r')
>>> pO = Point('pO')


A sphere with radius r centered on pO can be instantiated with:

>>> WrappingSphere(r, pO)


WrappingCylinder

Cylindrical geometry where the wrapping direction can be defined.

geodesic_end_vectors(point_1, point_2)[source]#

The vectors parallel to the geodesic at the two end points.

Parameters:

point_1 : Point

The point from which the geodesic originates.

point_2 : Point

The point at which the geodesic terminates.

geodesic_length(point_1, point_2)[source]#

Returns the shortest distance between two points on the sphere’s surface.

Parameters:

point_1 : Point

Point from which the geodesic length should be calculated.

point_2 : Point

Point to which the geodesic length should be calculated.

Explanation

The geodesic length, i.e. the shortest arc along the surface of a sphere, connecting two points can be calculated using the formula:

$l = \arccos\left(\mathbf{v}_1 \cdot \mathbf{v}_2\right)$

where $$\mathbf{v}_1$$ and $$\mathbf{v}_2$$ are the unit vectors from the sphere’s center to the first and second points on the sphere’s surface respectively. Note that the actual path that the geodesic will take is undefined when the two points are directly opposite one another.

Examples

A geodesic length can only be calculated between two points on the sphere’s surface. Firstly, a WrappingSphere instance must be created along with two points that will lie on its surface:

>>> from sympy import symbols
>>> from sympy.physics.mechanics import (Point, ReferenceFrame,
...     WrappingSphere)
>>> N = ReferenceFrame('N')
>>> r = symbols('r')
>>> pO = Point('pO')
>>> pO.set_vel(N, 0)
>>> sphere = WrappingSphere(r, pO)
>>> p1 = Point('p1')
>>> p2 = Point('p2')


Let’s assume that p1 lies at a distance of r in the N.x direction from pO and that p2 is located on the sphere’s surface in the N.y + N.z direction from pO. These positions can be set with:

>>> p1.set_pos(pO, r*N.x)
>>> p1.pos_from(pO)
r*N.x
>>> p2.set_pos(pO, r*(N.y + N.z).normalize())
>>> p2.pos_from(pO)
sqrt(2)*r/2*N.y + sqrt(2)*r/2*N.z


The geodesic length, which is in this case is a quarter of the sphere’s circumference, can be calculated using the geodesic_length method:

>>> sphere.geodesic_length(p1, p2)
pi*r/2


If the geodesic_length method is passed an argument, the Point that doesn’t lie on the sphere’s surface then a ValueError is raised because it’s not possible to calculate a value in this case.

property point#

A point on which the sphere is centered.

point_on_surface(point)[source]#

Returns True if a point is on the sphere’s surface.

Parameters:

point : Point

The point for which it’s to be ascertained if it’s on the sphere’s surface or not. This point’s position relative to the sphere’s center must be a simple expression involving the radius of the sphere, otherwise this check will likely not work.