I want to plot the following equation

((1/2)*x*tan(log((1/5)*(2*(x^2+y^2)))) = y

I tried:

with(plots, implicitplot)

implicitplot((1/2)*x*tan(log((1/5)*(2*(x^2+y^2)))) = y, x = -5 .. 5, y =-5..5, numpoints = 1000, gridrefine = 6, color = red)

but result is strange... 

I want to plot the position of Spherical pendulum. there are differential equation for spherical pendulum in spherical coordinates

sys := {((D@@2)(phi))(t) = -2*(D(phi))(t)*(D(theta))(t)*cos(theta(t))/sin(phi(t)),
((D@@2)(theta))(t) = (D(phi))(t)^2*cos(theta(t))*sin(theta(t))-9.8*sin(theta(t))}

with initial conditions

theta(0) = (1/2)*Pi, (D(theta))(0) = 0, phi(0) = (1/2)*Pi, (D(phi))(0) = 1

I tried:

eq := dsolve([((D@@2)(theta))(t) = (D(phi))(t)^2*cos(theta(t))-9.8*sin(theta(t)),
((D@@2)(phi))(t) = -2*(D(phi))(t)*(D(theta))(t)*cos(theta(t))/sin(phi(t)),
theta(0) = (1/2)*Pi, (D(theta))(0) = 0, phi(0) = (1/2)*Pi, (D(phi))(0) = 1],numeric)

how to change coordinates

x(t) = sin(theta(t))*cos(phi(t))
y(t) = sin(theta(t))*sin(phi(t))
z(t) = cos(theta(t))

and how to plot it from t=0 to 10?