what is promblem?

[별과바람]

# -*- coding: cp949 -*-

from visual import *

e_const = 9.e9 

e_charge = -1.6e-19

distance = 5.3e-11

q1 = sphere(pos=(-0.5*distance, 0, 0), radius=0.1*distance, color=color.blue) 

q2 = sphere(pos=scene.mouse.getclick().pos, radius=0.1*distance, color=color.red) 

q1.q = e_charge 

q2.q = -e_charge 

f_arrow = arrow(pos=q2.pos, color=color.yellow, axis=(0,0,0),

                shaftwidth=q2.radius/4.)

while True :

         q2.pos = scene.mouse.getclick().pos

         f_arrow.pos = q2.pos

         r_vec = q2.pos - q1.pos 

         r_mag = mag(r_vec) 

         r_hat = norm(r_vec)

         force = (e_const*q1.q*q2.q*r_hat)/(r_mag)**2

         force_mag = mag(force)

         force_hat = norm(force) 

         print force_mag, force_hat

         scale_factor = 0.5*r_mag/force_mag 

         f_arrow.axis = scale_factor*force

hello, I`m korean, so i can`t English very well.

Question: I made program that show you electric force arrow whenever you click anywhere but if i click anywhere , the arrow is not appear only first click. Since second click, all is good. what is problem??

         

[Bruce Sherwood]

The problem was that when you say q2 = sphere(pos=scene.mouse.getclick().pos) there is a wait for a click before proceeding to the next statement, and then the arrow is created with zero length. Here is a slightly modifies program that works:

# -*- coding: cp949 -*-

from visual import *

e_const = 9.e9 

e_charge = -1.6e-19

distance = 5.3e-11

q1 = sphere(pos=(-0.5*distance, 0, 0), radius=0.1*distance, color=color.blue)

q2 = None

q1.q = e_charge  

f_arrow = arrow(color=color.yellow, axis=(0,0,0), shaftwidth=q1.radius/4.)

while True :

         pos = scene.mouse.getclick().pos

         if q2 is None:

             q2 = sphere(radius=q1.radius, color=color.red, q=-q1.q)

         q2.pos = pos