def test_apply_force():
f, g = symbols('f g')
q, x, v1, v2 = dynamicsymbols('q x v1 v2')
P1 = Point('P1')
P2 = Point('P2')
B1 = Body('B1')
B2 = Body('B2')
N = ReferenceFrame('N')
P1.set_vel(B1.frame, v1 * B1.x)
P2.set_vel(B2.frame, v2 * B2.x)
force = f * q * N.z # time varying force
B1.apply_force(force, P1, B2,
P2) #applying equal and opposite force on moving points
assert B1.loads == [(P1, force)]
assert B2.loads == [(P2, -force)]
g1 = B1.mass * g * N.y
g2 = B2.mass * g * N.y
B1.apply_force(g1) #applying gravity on B1 masscenter
B2.apply_force(g2) #applying gravity on B2 masscenter
assert B1.loads == [(P1, force), (B1.masscenter, g1)]
assert B2.loads == [(P2, -force), (B2.masscenter, g2)]
force2 = x * N.x
B1.apply_force(
force2, reaction_body=B2) #Applying time varying force on masscenter
assert B1.loads == [(P1, force), (B1.masscenter, force2 + g1)]
assert B2.loads == [(P2, -force), (B2.masscenter, -force2 + g2)]
def test_auto_vel_derivative():
q1, q2 = dynamicsymbols('q1:3')
u1, u2 = dynamicsymbols('u1:3', 1)
A = ReferenceFrame('A')
B = ReferenceFrame('B')
C = ReferenceFrame('C')
B.orient_axis(A, A.z, q1)
B.set_ang_vel(A, u1 * A.z)
C.orient_axis(B, B.z, q2)
C.set_ang_vel(B, u2 * B.z)
Am = Point('Am')
Am.set_vel(A, 0)
Bm = Point('Bm')
Bm.set_pos(Am, B.x)
Bm.set_vel(B, 0)
Bm.set_vel(C, 0)
Cm = Point('Cm')
Cm.set_pos(Bm, C.x)
Cm.set_vel(C, 0)
temp = Cm._vel_dict.copy()
assert Cm.vel(A) == (u1 * B.y + (u1 + u2) * C.y)
Cm._vel_dict = temp
Cm.v2pt_theory(Bm, B, C)
assert Cm.vel(A) == (u1 * B.y + (u1 + u2) * C.y)
def test_point_funcs():
q, q2 = dynamicsymbols("q q2")
qd, q2d = dynamicsymbols("q q2", 1)
qdd, q2dd = dynamicsymbols("q q2", 2)
N = ReferenceFrame("N")
B = ReferenceFrame("B")
B.set_ang_vel(N, 5 * B.y)
O = Point("O")
P = O.locatenew("P", q * B.x)
assert P.pos_from(O) == q * B.x
P.set_vel(B, qd * B.x + q2d * B.y)
assert P.vel(B) == qd * B.x + q2d * B.y
O.set_vel(N, 0)
assert O.vel(N) == 0
assert P.a1pt_theory(O, N, B) == ((-25 * q + qdd) * B.x + (q2dd) * B.y +
(-10 * qd) * B.z)
B = N.orientnew("B", "Axis", [q, N.z])
O = Point("O")
P = O.locatenew("P", 10 * B.x)
O.set_vel(N, 5 * N.x)
assert O.vel(N) == 5 * N.x
assert P.a2pt_theory(O, N, B) == (-10 * qd**2) * B.x + (10 * qdd) * B.y
B.set_ang_vel(N, 5 * B.y)
O = Point("O")
P = O.locatenew("P", q * B.x)
P.set_vel(B, qd * B.x + q2d * B.y)
O.set_vel(N, 0)
assert P.v1pt_theory(O, N, B) == qd * B.x + q2d * B.y - 5 * q * B.z
def test_point_funcs():
q, q2 = dynamicsymbols('q q2')
qd, q2d = dynamicsymbols('q q2', 1)
qdd, q2dd = dynamicsymbols('q q2', 2)
N = ReferenceFrame('N')
B = ReferenceFrame('B')
B.set_ang_vel(N, 5 * B.y)
O = Point('O')
P = O.locatenew('P', q * B.x)
assert P.pos_from(O) == q * B.x
P.set_vel(B, qd * B.x + q2d * B.y)
assert P.vel(B) == qd * B.x + q2d * B.y
O.set_vel(N, 0)
assert O.vel(N) == 0
assert P.a1pt_theory(O, N, B) == ((-25 * q + qdd) * B.x + (q2dd) * B.y +
(-10 * qd) * B.z)
B = N.orientnew('B', 'Axis', [q, N.z])
O = Point('O')
P = O.locatenew('P', 10 * B.x)
O.set_vel(N, 5 * N.x)
assert O.vel(N) == 5 * N.x
assert P.a2pt_theory(O, N, B) == (-10 * qd**2) * B.x + (10 * qdd) * B.y
B.set_ang_vel(N, 5 * B.y)
O = Point('O')
P = O.locatenew('P', q * B.x)
P.set_vel(B, qd * B.x + q2d * B.y)
O.set_vel(N, 0)
assert P.v1pt_theory(O, N, B) == qd * B.x + q2d * B.y - 5 * q * B.z
def test_point_vel(): #Basic functionality
q1, q2 = dynamicsymbols('q1 q2')
N = ReferenceFrame('N')
B = ReferenceFrame('B')
Q = Point('Q')
O = Point('O')
Q.set_pos(O, q1 * N.x)
raises(ValueError, lambda: Q.vel(N)) # Velocity of O in N is not defined
O.set_vel(N, q2 * N.y)
assert O.vel(N) == q2 * N.y
raises(ValueError, lambda: O.vel(B)) #Velocity of O is not defined in B
def test_auto_point_vel_if_tree_has_vel_but_inappropriate_pos_vector():
q1, q2 = dynamicsymbols('q1 q2')
B = ReferenceFrame('B')
S = ReferenceFrame('S')
P = Point('P')
P.set_vel(B, q1 * B.x)
P1 = Point('P1')
P1.set_pos(P, S.y)
raises(ValueError,
lambda: P1.vel(B)) # P1.pos_from(P) can't be expressed in B
raises(ValueError, lambda: P1.vel(S)) # P.vel(S) not defined
def test_auto_vel_dont_overwrite():
t = dynamicsymbols._t
q1, q2, u1 = dynamicsymbols('q1, q2, u1')
N = ReferenceFrame('N')
P = Point('P1')
P.set_vel(N, u1 * N.x)
P1 = Point('P1')
P1.set_pos(P, q2 * N.y)
assert P1.vel(N) == q2.diff(t) * N.y + u1 * N.x
assert P.vel(N) == u1 * N.x
P1.set_vel(N, u1 * N.z)
assert P1.vel(N) == u1 * N.z
def test_auto_point_vel_connected_frames():
t = dynamicsymbols._t
q, q1, q2, u = dynamicsymbols('q q1 q2 u')
N = ReferenceFrame('N')
B = ReferenceFrame('B')
O = Point('O')
O.set_vel(N, u * N.x)
P = Point('P')
P.set_pos(O, q1 * N.x + q2 * B.y)
raises(ValueError, lambda: P.vel(N))
N.orient(B, 'Axis', (q, B.x))
assert P.vel(
N) == (u + q1.diff(t)) * N.x + q2.diff(t) * B.y - q2 * q.diff(t) * B.z
def test_auto_point_vel_multiple_point_path():
t = dynamicsymbols._t
q1, q2 = dynamicsymbols('q1 q2')
B = ReferenceFrame('B')
P = Point('P')
P.set_vel(B, q1 * B.x)
P1 = Point('P1')
P1.set_pos(P, q2 * B.y)
P1.set_vel(B, q1 * B.z)
P2 = Point('P2')
P2.set_pos(P1, q1 * B.z)
P3 = Point('P3')
P3.set_pos(P2, 10 * q1 * B.y)
assert P3.vel(B) == 10 * q1.diff(t) * B.y + (q1 + q1.diff(t)) * B.z
def test_auto_vel_cyclic_warning_arises():
P = Point('P')
P1 = Point('P1')
P2 = Point('P2')
P3 = Point('P3')
N = ReferenceFrame('N')
P.set_vel(N, N.x)
P1.set_pos(P, N.x)
P2.set_pos(P1, N.y)
P3.set_pos(P2, N.z)
P1.set_pos(P3, N.x + N.y)
with warnings.catch_warnings(
): #The path is cyclic at P1, thus a warning is raised
warnings.simplefilter("error")
raises(UserWarning, lambda: P2.vel(N))
def test_remove_load():
P1 = Point('P1')
P2 = Point('P2')
B = Body('B')
f1 = B.x
f2 = B.y
B.apply_force(f1, P1)
B.apply_force(f2, P2)
B.loads == [(P1, f1), (P2, f2)]
B.remove_load(P2)
B.loads == [(P1, f1)]
B.apply_torque(f1.cross(f2))
B.loads == [(P1, f1), (B.frame, f1.cross(f2))]
B.remove_load()
B.loads == [(P1, f1)]
def test_scalar_potential_difference():
origin = Point("O")
point1 = origin.locatenew("P1", 1 * R.x + 2 * R.y + 3 * R.z)
point2 = origin.locatenew("P2", 4 * R.x + 5 * R.y + 6 * R.z)
genericpointR = origin.locatenew("RP",
R[0] * R.x + R[1] * R.y + R[2] * R.z)
genericpointP = origin.locatenew("PP",
P[0] * P.x + P[1] * P.y + P[2] * P.z)
assert scalar_potential_difference(S.Zero, R, point1, point2, origin) == 0
assert (scalar_potential_difference(scalar_field, R, origin, genericpointR,
origin) == scalar_field)
assert (scalar_potential_difference(grad_field, R, origin, genericpointR,
origin) == scalar_field)
assert scalar_potential_difference(grad_field, R, point1, point2,
origin) == 948
assert (scalar_potential_difference(
R[1] * R[2] * R.x + R[0] * R[2] * R.y + R[0] * R[1] * R.z,
R,
point1,
genericpointR,
origin,
) == R[0] * R[1] * R[2] - 6)
potential_diff_P = (2 * P[2] * (P[0] * sin(q) + P[1] * cos(q)) *
(P[0] * cos(q) - P[1] * sin(q))**2)
assert (scalar_potential_difference(grad_field, P, origin, genericpointP,
origin).simplify() == potential_diff_P)
def test_partial_velocity():
q1, q2, q3, u1, u2, u3 = dynamicsymbols("q1 q2 q3 u1 u2 u3")
u4, u5 = dynamicsymbols("u4, u5")
r = symbols("r")
N = ReferenceFrame("N")
Y = N.orientnew("Y", "Axis", [q1, N.z])
L = Y.orientnew("L", "Axis", [q2, Y.x])
R = L.orientnew("R", "Axis", [q3, L.y])
R.set_ang_vel(N, u1 * L.x + u2 * L.y + u3 * L.z)
C = Point("C")
C.set_vel(N, u4 * L.x + u5 * (Y.z ^ L.x))
Dmc = C.locatenew("Dmc", r * L.z)
Dmc.v2pt_theory(C, N, R)
vel_list = [Dmc.vel(N), C.vel(N), R.ang_vel_in(N)]
u_list = [u1, u2, u3, u4, u5]
assert partial_velocity(vel_list, u_list, N) == [
[-r * L.y, r * L.x, 0, L.x,
cos(q2) * L.y - sin(q2) * L.z],
[0, 0, 0, L.x, cos(q2) * L.y - sin(q2) * L.z],
[L.x, L.y, L.z, 0, 0],
]
# Make sure that partial velocities can be computed regardless if the
# orientation between frames is defined or not.
A = ReferenceFrame("A")
B = ReferenceFrame("B")
v = u4 * A.x + u5 * B.y
assert partial_velocity((v, ), (u4, u5), A) == [[A.x, B.y]]
raises(TypeError, lambda: partial_velocity(Dmc.vel(N), u_list, N))
raises(TypeError, lambda: partial_velocity(vel_list, u1, N))
def test_body_add_force():
# Body with RigidBody.
rigidbody_masscenter = Point('rigidbody_masscenter')
rigidbody_mass = Symbol('rigidbody_mass')
rigidbody_frame = ReferenceFrame('rigidbody_frame')
body_inertia = inertia(rigidbody_frame, 1, 0, 0)
rigid_body = Body('rigidbody_body', rigidbody_masscenter, rigidbody_mass,
rigidbody_frame, body_inertia)
l = Symbol('l')
Fa = Symbol('Fa')
point = rigid_body.masscenter.locatenew('rigidbody_body_point0',
l * rigid_body.frame.x)
point.set_vel(rigid_body.frame, 0)
force_vector = Fa * rigid_body.frame.z
# apply_force with point
rigid_body.apply_force(force_vector, point)
assert len(rigid_body.loads) == 1
force_point = rigid_body.loads[0][0]
frame = rigid_body.frame
assert force_point.vel(frame) == point.vel(frame)
assert force_point.pos_from(force_point) == point.pos_from(force_point)
assert rigid_body.loads[0][1] == force_vector
# apply_force without point
rigid_body.apply_force(force_vector)
assert len(rigid_body.loads) == 2
assert rigid_body.loads[1][1] == force_vector
# passing something else than point
raises(TypeError, lambda: rigid_body.apply_force(force_vector, 0))
raises(TypeError, lambda: rigid_body.apply_force(0))
def test_custom_rigid_body():
# Body with RigidBody.
rigidbody_masscenter = Point("rigidbody_masscenter")
rigidbody_mass = Symbol("rigidbody_mass")
rigidbody_frame = ReferenceFrame("rigidbody_frame")
body_inertia = inertia(rigidbody_frame, 1, 0, 0)
rigid_body = Body(
"rigidbody_body",
rigidbody_masscenter,
rigidbody_mass,
rigidbody_frame,
body_inertia,
)
com = rigid_body.masscenter
frame = rigid_body.frame
rigidbody_masscenter.set_vel(rigidbody_frame, 0)
assert com.vel(frame) == rigidbody_masscenter.vel(frame)
assert com.pos_from(com) == rigidbody_masscenter.pos_from(com)
assert rigid_body.mass == rigidbody_mass
assert rigid_body.inertia == (body_inertia, rigidbody_masscenter)
assert hasattr(rigid_body, "masscenter")
assert hasattr(rigid_body, "mass")
assert hasattr(rigid_body, "frame")
assert hasattr(rigid_body, "inertia")
def test_partial_velocity():
q1, q2, q3, u1, u2, u3 = dynamicsymbols('q1 q2 q3 u1 u2 u3')
u4, u5 = dynamicsymbols('u4, u5')
r = symbols('r')
N = ReferenceFrame('N')
Y = N.orientnew('Y', 'Axis', [q1, N.z])
L = Y.orientnew('L', 'Axis', [q2, Y.x])
R = L.orientnew('R', 'Axis', [q3, L.y])
R.set_ang_vel(N, u1 * L.x + u2 * L.y + u3 * L.z)
C = Point('C')
C.set_vel(N, u4 * L.x + u5 * (Y.z ^ L.x))
Dmc = C.locatenew('Dmc', r * L.z)
Dmc.v2pt_theory(C, N, R)
vel_list = [Dmc.vel(N), C.vel(N), R.ang_vel_in(N)]
u_list = [u1, u2, u3, u4, u5]
assert (partial_velocity(vel_list, u_list, N) == [[
-r * L.y, r * L.x, 0, L.x,
cos(q2) * L.y - sin(q2) * L.z
], [0, 0, 0, L.x, cos(q2) * L.y - sin(q2) * L.z], [L.x, L.y, L.z, 0, 0]])
# Make sure that partial velocities can be computed regardless if the
# orientation between frames is defined or not.
A = ReferenceFrame('A')
B = ReferenceFrame('B')
v = u4 * A.x + u5 * B.y
assert partial_velocity((v, ), (u4, u5), A) == [[A.x, B.y]]
raises(TypeError, lambda: partial_velocity(Dmc.vel(N), u_list, N))
raises(TypeError, lambda: partial_velocity(vel_list, u1, N))
def test_scalar_potential_difference():
origin = Point('O')
point1 = origin.locatenew('P1', 1*R.x + 2*R.y + 3*R.z)
point2 = origin.locatenew('P2', 4*R.x + 5*R.y + 6*R.z)
genericpointR = origin.locatenew('RP', R[0]*R.x + R[1]*R.y + R[2]*R.z)
genericpointP = origin.locatenew('PP', P[0]*P.x + P[1]*P.y + P[2]*P.z)
assert scalar_potential_difference(S(0), R, point1, point2, \
origin) == 0
assert scalar_potential_difference(scalar_field, R, origin, \
genericpointR, origin) == \
scalar_field
assert scalar_potential_difference(grad_field, R, origin, \
genericpointR, origin) == \
scalar_field
assert scalar_potential_difference(grad_field, R, point1, point2,
origin) == 948
assert scalar_potential_difference(R[1]*R[2]*R.x + R[0]*R[2]*R.y + \
R[0]*R[1]*R.z, R, point1,
genericpointR, origin) == \
R[0]*R[1]*R[2] - 6
potential_diff_P = 2*P[2]*(P[0]*sin(q) + P[1]*cos(q))*\
(P[0]*cos(q) - P[1]*sin(q))**2
assert scalar_potential_difference(grad_field, P, origin, \
genericpointP, \
origin).simplify() == \
potential_diff_P
def test_auto_point_vel_multiple_paths_warning_arises():
q, u = dynamicsymbols('q u')
N = ReferenceFrame('N')
O = Point('O')
P = Point('P')
Q = Point('Q')
R = Point('R')
P.set_vel(N, u * N.x)
Q.set_vel(N, u * N.y)
R.set_vel(N, u * N.z)
O.set_pos(P, q * N.z)
O.set_pos(Q, q * N.y)
O.set_pos(R, q * N.x)
with warnings.catch_warnings(
): #There are two possible paths in this point tree, thus a warning is raised
warnings.simplefilter("error")
raises(UserWarning, lambda: O.vel(N))
def test_auto_point_vel_shortest_path():
t = dynamicsymbols._t
q1, q2, u1, u2 = dynamicsymbols('q1 q2 u1 u2')
B = ReferenceFrame('B')
P = Point('P')
P.set_vel(B, u1 * B.x)
P1 = Point('P1')
P1.set_pos(P, q2 * B.y)
P1.set_vel(B, q1 * B.z)
P2 = Point('P2')
P2.set_pos(P1, q1 * B.z)
P3 = Point('P3')
P3.set_pos(P2, 10 * q1 * B.y)
P4 = Point('P4')
P4.set_pos(P3, q1 * B.x)
O = Point('O')
O.set_vel(B, u2 * B.y)
O1 = Point('O1')
O1.set_pos(O, q2 * B.z)
P4.set_pos(O1, q1 * B.x + q2 * B.z)
with warnings.catch_warnings(
): #There are two possible paths in this point tree, thus a warning is raised
warnings.simplefilter('error')
with ignore_warnings(UserWarning):
assert P4.vel(
B) == q1.diff(t) * B.x + u2 * B.y + 2 * q2.diff(t) * B.z
def test_auto_point_vel():
t = dynamicsymbols._t
q1, q2 = dynamicsymbols('q1 q2')
N = ReferenceFrame('N')
B = ReferenceFrame('B')
O = Point('O')
Q = Point('Q')
Q.set_pos(O, q1 * N.x)
O.set_vel(N, q2 * N.y)
assert Q.vel(N) == q1.diff(t) * N.x + q2 * N.y # Velocity of Q using O
P1 = Point('P1')
P1.set_pos(O, q1 * B.x)
P2 = Point('P2')
P2.set_pos(P1, q2 * B.z)
raises(ValueError, lambda: P2.vel(B)
) # O's velocity is defined in different frame, and no
#point in between has its velocity defined
raises(ValueError, lambda: P2.vel(N)) # Velocity of O not defined in N
def test_clear_load():
a = symbols('a')
P = Point('P')
B = Body('B')
force = a * B.z
B.apply_force(force, P)
assert B.loads == [(P, force)]
B.clear_loads()
assert B.loads == []
def test_apply_force_multiple_one_point():
a, b = symbols('a b')
P = Point('P')
B = Body('B')
f1 = a * B.x
f2 = b * B.y
B.apply_force(f1, P)
assert B.loads == [(P, f1)]
B.apply_force(f2, P)
assert B.loads == [(P, f1 + f2)]
def test_auto_vel_cyclic_warning_msg():
P = Point('P')
P1 = Point('P1')
P2 = Point('P2')
P3 = Point('P3')
N = ReferenceFrame('N')
P.set_vel(N, N.x)
P1.set_pos(P, N.x)
P2.set_pos(P1, N.y)
P3.set_pos(P2, N.z)
P1.set_pos(P3, N.x + N.y)
with warnings.catch_warnings(
record=True
) as w: #The path is cyclic at P1, thus a warning is raised
warnings.simplefilter("always")
P2.vel(N)
assert issubclass(w[-1].category, UserWarning)
assert 'Kinematic loops are defined among the positions of points. This is likely not desired and may cause errors in your calculations.' in str(
w[-1].message)
def test_auto_vel_multiple_path_warning_msg():
N = ReferenceFrame('N')
O = Point('O')
P = Point('P')
Q = Point('Q')
P.set_vel(N, N.x)
Q.set_vel(N, N.y)
O.set_pos(P, N.z)
O.set_pos(Q, N.y)
with warnings.catch_warnings(
record=True
) as w: #There are two possible paths in this point tree, thus a warning is raised
warnings.simplefilter("always")
O.vel(N)
assert issubclass(w[-1].category, UserWarning)
assert 'Velocity automatically calculated based on point' in str(
w[-1].message)
assert 'Velocities from these points are not necessarily the same. This may cause errors in your calculations.' in str(
w[-1].message)
def test_point_pos():
q = dynamicsymbols("q")
N = ReferenceFrame("N")
B = N.orientnew("B", "Axis", [q, N.z])
O = Point("O")
P = O.locatenew("P", 10 * N.x + 5 * B.x)
assert P.pos_from(O) == 10 * N.x + 5 * B.x
Q = P.locatenew("Q", 10 * N.y + 5 * B.y)
assert Q.pos_from(P) == 10 * N.y + 5 * B.y
assert Q.pos_from(O) == 10 * N.x + 10 * N.y + 5 * B.x + 5 * B.y
assert O.pos_from(Q) == -10 * N.x - 10 * N.y - 5 * B.x - 5 * B.y
def test_point_pos():
q = dynamicsymbols('q')
N = ReferenceFrame('N')
B = N.orientnew('B', 'Axis', [q, N.z])
O = Point('O')
P = O.locatenew('P', 10 * N.x + 5 * B.x)
assert P.pos_from(O) == 10 * N.x + 5 * B.x
Q = P.locatenew('Q', 10 * N.y + 5 * B.y)
assert Q.pos_from(P) == 10 * N.y + 5 * B.y
assert Q.pos_from(O) == 10 * N.x + 10 * N.y + 5 * B.x + 5 * B.y
assert O.pos_from(Q) == -10 * N.x - 10 * N.y - 5 * B.x - 5 * B.y
def test_point_a2pt_theorys():
q = dynamicsymbols('q')
qd = dynamicsymbols('q', 1)
qdd = dynamicsymbols('q', 2)
N = ReferenceFrame('N')
B = N.orientnew('B', 'Axis', [q, N.z])
O = Point('O')
P = O.locatenew('P', 0)
O.set_vel(N, 0)
assert P.a2pt_theory(O, N, B) == 0
P.set_pos(O, B.x)
assert P.a2pt_theory(O, N, B) == (-qd**2) * B.x + (qdd) * B.y
def test_point_a2pt_theorys():
q = dynamicsymbols("q")
qd = dynamicsymbols("q", 1)
qdd = dynamicsymbols("q", 2)
N = ReferenceFrame("N")
B = N.orientnew("B", "Axis", [q, N.z])
O = Point("O")
P = O.locatenew("P", 0)
O.set_vel(N, 0)
assert P.a2pt_theory(O, N, B) == 0
P.set_pos(O, B.x)
assert P.a2pt_theory(O, N, B) == (-(qd**2)) * B.x + (qdd) * B.y
def test_point_v2pt_theorys():
q = dynamicsymbols('q')
qd = dynamicsymbols('q', 1)
N = ReferenceFrame('N')
B = N.orientnew('B', 'Axis', [q, N.z])
O = Point('O')
P = O.locatenew('P', 0)
O.set_vel(N, 0)
assert P.v2pt_theory(O, N, B) == 0
P = O.locatenew('P', B.x)
assert P.v2pt_theory(O, N, B) == (qd * B.z ^ B.x)
O.set_vel(N, N.x)
assert P.v2pt_theory(O, N, B) == N.x + qd * B.y
def test_point_v2pt_theorys():
q = dynamicsymbols("q")
qd = dynamicsymbols("q", 1)
N = ReferenceFrame("N")
B = N.orientnew("B", "Axis", [q, N.z])
O = Point("O")
P = O.locatenew("P", 0)
O.set_vel(N, 0)
assert P.v2pt_theory(O, N, B) == 0
P = O.locatenew("P", B.x)
assert P.v2pt_theory(O, N, B) == (qd * B.z ^ B.x)
O.set_vel(N, N.x)
assert P.v2pt_theory(O, N, B) == N.x + qd * B.y
