Understanding Python Programs

neilk32

I have some Python programs for showing some physics simulations, I haven't looked at python in over a year and then only spent a very short time on it. I was wondering if someone with knowledge of python could help me with understanding them. They are ran in vIdle for vPython.

Bouncing Projectile Code :


from visual import *

floor = box(length=4, height=0.5, width=4, color=color.blue)

ball = sphere(pos=(0,4,0), color=color.red, radius=1)
ball.velocity = vector(0,-1,0)

dt = 0.01
while 1:
rate(100)
ball.pos = ball.pos + ball.velocity*dt
if ball.y < 1:
ball.velocity.y = -ball.velocity.y
else:
ball.velocity.y = ball.velocity.y - 9.8*dt


Circular Motion Code:


from visual import *

ball = sphere(pos=(0,10,0), color=color.magenta, radius=0.5)
string = cylinder(pos=(0,0,0), axis=(0,10,0), color=color.white, radius=0.05)
PI = 3.1415926535
radius = 10
t=0
scene.autoscale = 0
while 1:
rate(100)
t = t + 0.01
ball.pos = (radius * sin (PI*t - 0.5 * sin(PI*t)),radius * cos (PI*t - 0.5 * sin(PI*t)),0)
string.axis = (ball.pos.x,ball.pos.y,0)
sphere(pos=(10 * sin (PI*t - 0.5 * sin(PI*t)),10 * cos (PI*t - 0.5 * sin(PI*t)),0),color=color.blue, radius=0.1)

Multiple Balls Code:


from visual import *
from random import uniform

thk = 0.3
side = 4.0
s2 = 2*side - thk
s3 = 2*side + thk

wallR = box (pos=vector( side, 0, 0), length=thk, height=s2, width=s3, color = color.green)
wallL = box (pos=vector(-side, 0, 0), length=thk, height=s2, width=s3, color = color.green)
wallB = box (pos=vector(0, -side, 0), length=s3, height=thk, width=s3, color = color.green)
wallT = box (pos=vector(0, side, 0), length=s3, height=thk, width=s3, color = color.green)
wallBK = box(pos=vector(0, 0, -side), length=s2, height=s2, width=thk, color = color.white)

no_particles=10
ball_radius= 0.5
maxpos=side-.5*thk-ball_radius
maxv=1.0

ball_list=[]
for i in arange(no_particles):
ball=sphere(color=color.yellow,radius=ball_radius)
ball.pos=maxpos*vector(uniform(-1,1),uniform(-1,1),uniform(-1,1))
ball.velocity=maxv*vector(uniform(-1,1),uniform(-1,1),uniform(-1,1))
ball_list.append(ball)

timestep = 0.05

while (1==1):
rate(100)
for ball in ball_list:
ball.pos = ball.pos + ball.velocity*timestep
if ball.x > maxpos:
ball.velocity.x = -ball.velocity.x
ball.x=2*maxpos-ball.x
if ball.x < -maxpos:
ball.velocity.x = -ball.velocity.x
ball.x=-2*maxpos-ball.x
if ball.y > maxpos:
ball.velocity.y = -ball.velocity.y
ball.y=2*maxpos-ball.y
if ball.y < -maxpos:
ball.velocity.y = -ball.velocity.y
ball.y=-2*maxpos-ball.y
if ball.z > maxpos:
ball.velocity.z = -ball.velocity.z
ball.z=2*maxpos-ball.z
if ball.z < -maxpos:
ball.velocity.z = -ball.velocity.z
ball.z=-2*maxpos-ball.z

for i in range(no_particles):
for j in range(i+1,no_particles):
distance=mag(ball_list.pos-ball_list[j].pos)
if distance<(ball_list.radius+ball_list[j].radius):
direction=norm(ball_list[j].pos-ball_list.pos)
vi=dot(ball_list.velocity,direction)
vj=dot(ball_list[j].velocity,direction)
exchange=vj-vi
ball_list.velocity=ball_list.velocity + exchange*direction
ball_list[j].velocity=ball_list[j].velocity - exchange*direction
overlap=2*ball_radius-distance
ball_list.pos=ball_list.pos - overlap*direction
ball_list[j].pos=ball_list[j].pos + overlap*direction


Thanks in advance for help.

-Trek-

Interesting add on, never knew existed, could be very useful , there is documentation for visual here http://www.vpython.org/index.html

They explain the first program in your post in detail also if it helps....

Here is a complete VPython program that produces a 3D animation of a red ball bouncing on a blue floor as shown above. Note that in the "while" loop there are no graphics commands, just computations to update the position of the ball and check whether it hits the floor. The 3D animation is a side effect of these computations.

from visual import *

floor = box (pos=(0,0,0), length=4, height=0.5, width=4, color=color.blue)
ball = sphere (pos=(0,4,0), radius=1, color=color.red)
ball.velocity = vector(0,-1,0)
dt = 0.01

while 1:
    rate (100)
    ball.pos = ball.pos + ball.velocity*dt
    if ball.y < ball.radius:
        ball.velocity.y = abs(ball.velocity.y)
    else:
        ball.velocity.y = ball.velocity.y - 9.8*dt

The program starts by importing the module "visual" which enables 3D graphics.

A box and a sphere are created and given names "floor" and "ball" so one can refer to these objects.

The ball is given a vector velocity.

In the while loop, "rate(100" has the effect "no more than 100 loops per second on fast computers."

The ball's velocity is used to update its position, in a single vector statement that updates x, y, and z.

Visual periodically use the current values of the objects' attributes, including ball.pos, to paint a 3D picture.

The program checks for the ball touching the floor and if necessary reverses the y-component of velocity, otherwise the velocity is updated due to the gravitational force.