Handouts: Graphics Reference

Make a ball that bounces as if under the influence of gravity. To do this, adapt code for a regular bouncing ball, but each frame have the y velocity increase downward by a small amount to simulate the pull of gravity. Additionally, each time the ball bounces off the bottom, decrease the y velocity by a certain percentage. (Note: the gray path in the image above is just an illustration - the working program will just have a black ball bouncing on the screen)

## Solution

"""
File: gravity_ball.py
-------------------
A ball bounces as if due to gravity.  Specifically, each frame the y velocity increases
downward to simulate gravity, and each time it bounces it loses some of its y velocity
to simulate damping.
"""

import time
from graphics import Canvas

BALL_DIAMETER = 20
ANIMATION_DELAY_SECONDS = 0.02

# The amount to add each frame to the y velocity to simulate the pull of gravity
GRAVITY = 0.5

# The amount by which to multiply the y velocity each time we bounce (simulates losing energy)
DAMPING = 0.7

def main():
canvas = Canvas()
canvas.set_canvas_title("Gravity Ball")

# draw a ball on the top left corner
ball = canvas.create_oval(0, 0, BALL_DIAMETER, BALL_DIAMETER)
canvas.set_color(ball, "black")

# variables for velocity
vx = 3
vy = 0

while True:
# update vy
vy += GRAVITY

# should the ball bounce?
if (canvas.get_top_y(ball) > (canvas.get_canvas_height() - canvas.get_height(ball))) and vy > 0:
# Dampen the velocity and invert the velocity
vy = vy * -DAMPING

canvas.move(ball, vx, vy)
time.sleep(ANIMATION_DELAY_SECONDS)
canvas.update()

canvas.mainloop()

if __name__ == "__main__":
main()