Bouncing Ball

By the end of this lab you'll be able to:

• Animate a turtle by updating its position in a loop using velocity variables
• Reverse velocity when the ball hits a wall (vx = -vx)
• Use turtle.tracer(0) and turtle.update() for smooth animation

A colored ball bounces around inside a rectangular boundary. Each frame, the ball moves by its velocity, and when it hits a wall, the velocity component reverses — simulating elastic collision.

Welcome to Bouncing Ball!

This is your first animated turtle program — the turtle moves continuously inside a loop, and you see smooth motion on screen! Let's code it together!

How It Works

Two variables vx and vy store the ball's velocity. Each step: x += vx, y += vy. If x hits the wall bounds, vx = -vx (reverse horizontal direction). If y hits the bounds, vy = -vy. turtle.tracer(0) turns off auto-drawing so we control when to refresh the screen.

Sample Code

import turtle
screen = turtle.Screen()
screen.tracer(0)

ball = turtle.Turtle()
ball.shape('circle')
ball.color('royalblue')
ball.penup()
ball.speed(0)

x, y = 0, 0
vx, vy = 3, 2
boundary = 150

for _ in range(400):
    x += vx
    y += vy
    if x > boundary or x < -boundary:
        vx = -vx
    if y > boundary or y < -boundary:
        vy = -vy
    ball.goto(x, y)
    screen.update()

What Do You Think Will Happen?

The ball starts at (0,0) with velocity (3,2). After bouncing off all four walls many times, will it return to (0,0)? Make your guess — then click Run to find out!

Try It Now

▶ Run ↻ Reset

The ball bounces around the box — reversing direction whenever it hits a wall. Were you right about returning to the origin?

How It Works

screen.tracer(0) tells turtle not to draw automatically after each command. screen.update() forces a single screen redraw. This gives smooth animation instead of flicker. The ball's position (x, y) is updated each step, and the boundary check x > boundary detects wall collisions.

Explanation Table

Line	What it does
screen.tracer(0)	Disable auto-update for smooth animation
vx, vy = 3, 2	Initial velocity vector
if x > boundary: vx = -vx	Reverse x velocity on wall hit
screen.update()	Redraw the screen (one frame)

Learning Check

Your Turn — Add a Trail

Add ball.pendown() before the loop to leave a trail. The ball's path will trace out a geometric pattern — predict what it looks like!

▶ Run ↻ Reset

The trail traces out a Lissajous-like figure! With vx=5, vy=3 (ratio 5:3), the path eventually returns to its starting point after tracing a complex geometric pattern.

Experiments

1. Make the ball leave a color trail. Change the color each time a wall is hit. You'll know it worked when the trail changes color at each bounce.

2. Two balls. Create a second turtle ball2 with a different starting velocity. You'll know it worked when two balls bounce independently.

3. Speed up over time. Add vx *= 1.002; vy *= 1.002 each step. You'll know it worked when the ball gradually bounces faster.

4. Gravity simulation. Add vy -= 0.1 each step and set if y < -boundary: vy = abs(vy) * 0.9. You'll know it worked when the ball falls and bounces like gravity.

Animated!

You wrote your first animation loop — position, velocity, collision detection! These are the building blocks of every physics-based video game. Up next: Clock Face — animated hands that show the current time.