PZ-35 · Simple Physics — Gravity & Jump

Simple Physics — Gravity & Jump

Every platform game from Mario to Celeste uses the same three-line physics recipe: add gravity to vertical velocity every frame, move the actor by that velocity, and stop it when it hits the floor. Today you build exactly that — and learn why the y-axis points downward makes a jump feel right when vy goes negative.

⏱ 1 hour📦 Concept lesson📚 After PZ-34💻 VS Code or Thonny

Learning Goals

3 min

By the end of this lesson you can:

Implement gravity by adding GRAVITY to vy every frame and moving the actor by vy.
Land an actor on a floor by checking actor.bottom >= FLOOR_Y and resetting vy = 0.
Make the actor jump by setting vy to a negative impulse value when the player presses a key — and only when the actor is on the ground.

Warm-Up · Which Way Is Down?

5 min

In Pygame Zero, (0, 0) is the top-left corner and y grows downward. Predict what happens to an actor with these values after two frames:

# start of frame 0
y = 100
vy = 5

# each frame:
vy = vy + 1
y = y + vy

Show the answer

Trace

Frame 0: vy = 5+1 = 6,  y = 100+6  = 106
Frame 1: vy = 6+1 = 7,  y = 106+7  = 113

The actor moves down the screen (y increases) and gets faster each frame — exactly like a falling object. That +1 each frame is gravity.

New Concept · Gravity, Landing, and Jumping

12 min

Think of vy as the actor's speed in the vertical direction — positive means moving down. Gravity is a constant pull that adds to vy every single frame, making the actor fall faster and faster until something stops it.

Gravity — three lines

GRAVITY = 0.5

def update():
    global vy
    vy += GRAVITY      # gravity pulls down (increases vy)
    player.y += vy     # move the actor

Landing on the floor

When the bottom of the actor touches the floor, snap it to the floor surface and zero the velocity:

FLOOR_Y = 500          # y-coordinate of the floor surface

def update():
    global vy, on_ground
    vy += GRAVITY
    player.y += vy
    if player.bottom >= FLOOR_Y:
        player.bottom = FLOOR_Y
        vy = 0
        on_ground = True
    else:
        on_ground = False

Jumping

A jump sets vy to a negative value — that pushes the actor upward (remember y grows down, so negative vy means moving up). Only allow a jump when the actor is already on the ground:

JUMP_FORCE = -12       # negative = upward

def on_key_down(key):
    global vy
    if key == keys.SPACE and on_ground:
        vy = JUMP_FORCE

Worked Example · A Jumping Ball

12 min

Aisyah wants to test gravity before building a full platform game. She draws a coloured circle (no image needed) that falls, lands on a floor line, and jumps when she presses SPACE. Save as jump_test.py:

# jump_test.py — gravity and jump demo

import pgzrun

WIDTH = 480
HEIGHT = 400
TITLE = "Gravity Demo"
GRAVITY = 0.5
JUMP_FORCE = -13
FLOOR_Y = 360

player_x = 240
player_y = 200
vy = 0.0
on_ground = False

def update():
    global player_y, vy, on_ground
    vy += GRAVITY
    player_y += vy
    if player_y + 20 >= FLOOR_Y:
        player_y = FLOOR_Y - 20
        vy = 0.0
        on_ground = True
    else:
        on_ground = False

def on_key_down(key):
    global vy
    if key == keys.SPACE and on_ground:
        vy = JUMP_FORCE

def draw():
    screen.fill("skyblue")
    screen.draw.filled_rect(
        Rect((0, FLOOR_Y), (WIDTH, HEIGHT - FLOOR_Y)), "saddlebrown"
    )
    screen.draw.filled_circle((int(player_x), int(player_y)), 20, "red")
    hint = "On ground — SPACE to jump" if on_ground else "In the air"
    screen.draw.text(hint, topleft=(10, 10), fontsize=22, color="white")

pgzrun.go()

Sky-blue window with a brown floor. The red circle rests on the floor; pressing SPACE launches it upward.

Press SPACE to launch the ball. Watch how it rises slowly, pauses at the top, then falls faster — that arc is gravity doing its job.

Try It Yourself

13 min

01 🟢 Tune gravity and jump force

Change GRAVITY to 0.3 and JUMP_FORCE to -10. Then try GRAVITY = 1.5 and JUMP_FORCE = -20. Describe in one sentence how the feel changes.

# just edit the two constants at the top
GRAVITY = 0.3
JUMP_FORCE = -10

02 🟡 Horizontal movement

Add left/right movement with LEFT and RIGHT keys. Keep the player inside the window by clamping player_x between 20 and WIDTH - 20.

WALK_SPEED = 4

def update():
    global player_x
    if keyboard.left:
        player_x -= WALK_SPEED
    if keyboard.right:
        player_x += WALK_SPEED
    player_x = max(20, min(WIDTH - 20, player_x))

03 🔴 Double jump

Allow one mid-air jump (a "double jump") by tracking how many jumps have been used since last landing. Reset the counter to 0 on landing, allow jumps while the counter is below 2.

Mini-Challenge · Debug Kavitha's Jump

8 min

Kavitha's character never stops falling — it sinks through the floor. Find the two bugs.

# kavitha_jump.py — buggy
GRAVITY = 0.5
FLOOR_Y = 360
player_y = 100
vy = 0.0
on_ground = False

def update():
    global player_y, vy, on_ground
    player_y += vy
    vy += GRAVITY
    if player_y >= FLOOR_Y:
        vy = 0.0
        on_ground = True
    else:
        on_ground = False

It works if…

the circle rests on the floor (doesn't sink),
and on_ground is True while standing

Show the fixed version

# kavitha_jump.py — fixed
GRAVITY = 0.5
FLOOR_Y = 360
player_y = 100
vy = 0.0
on_ground = False

def update():
    global player_y, vy, on_ground
    # Bug 1: gravity must be added BEFORE moving, not after
    vy += GRAVITY
    player_y += vy
    # Bug 2: player_y must be clamped to FLOOR_Y so it doesn't sink through
    if player_y + 20 >= FLOOR_Y:
        player_y = FLOOR_Y - 20
        vy = 0.0
        on_ground = True
    else:
        on_ground = False

Two bugs: gravity was added after moving (late update gives wrong arc), and the floor check forgot to snap player_y back so the actor kept sinking.

Recap

3 min

Gravity is just vy += GRAVITY every frame. The actor falls because each frame its vertical speed increases. Landing resets vy to zero and snaps the actor to the floor surface. Jumping sets vy to a negative impulse — upward in screen coordinates — and gravity pulls it back down automatically. Remember: y grows downward on screen, so upward motion means a smaller y value.

Vocabulary Card

vy (vertical velocity): How many pixels the actor moves downward per frame. Positive = falling; negative = rising.
GRAVITY: A small positive constant added to vy every frame, making the actor accelerate downward.
JUMP_FORCE: A negative value assigned to vy when the player jumps — it launches the actor upward.
on_ground: A boolean flag that is True only when the actor is touching the floor. Guards against infinite jumping.

Homework

4 min

Add a ceiling to the jump demo. When the top of the actor reaches y = 0 (the top edge of the window), immediately reverse vy to a small positive value (e.g. vy = 2) so the actor bounces off the ceiling instead of leaving the screen. Save as jump_ceiling.py.

Sample · `jump_ceiling.py`

# jump_ceiling.py — ceiling bounce added

import pgzrun

WIDTH = 480
HEIGHT = 400
GRAVITY = 0.5
JUMP_FORCE = -13
FLOOR_Y = 360
RADIUS = 20

player_x = 240
player_y = 200
vy = 0.0
on_ground = False

def update():
    global player_y, vy, on_ground
    vy += GRAVITY
    player_y += vy
    # floor landing
    if player_y + RADIUS >= FLOOR_Y:
        player_y = FLOOR_Y - RADIUS
        vy = 0.0
        on_ground = True
    else:
        on_ground = False
    # ceiling bounce
    if player_y - RADIUS <= 0:
        player_y = RADIUS
        vy = 2

def on_key_down(key):
    global vy
    if key == keys.SPACE and on_ground:
        vy = JUMP_FORCE

def draw():
    screen.fill("skyblue")
    screen.draw.filled_rect(Rect((0, FLOOR_Y), (WIDTH, HEIGHT - FLOOR_Y)), "saddlebrown")
    screen.draw.filled_circle((int(player_x), int(player_y)), RADIUS, "red")

pgzrun.go()

The ceiling check mirrors the floor check — clamp position, set a small positive vy to bounce away.

Simple Physics — Gravity & Jump

⏱ 1 hour📦 Concept lesson📚 After PZ-34💻 VS Code or Thonny

FLOOR_Y = 500 # y-coordinate of the floor surface def update(): global vy, on_ground vy += GRAVITY player.y += vy if player.bottom >= FLOOR_Y: player.bottom = FLOOR_Y vy = 0 on_ground = True else: on_ground = False

player_x = 240 player_y = 200 vy = 0.0 on_ground = False def update(): global player_y, vy, on_ground vy += GRAVITY player_y += vy if player_y + 20 >= FLOOR_Y: player_y = FLOOR_Y - 20 vy = 0.0 on_ground = True else: on_ground = False def on_key_down(key): global vy if key == keys.SPACE and on_ground: vy = JUMP_FORCE def draw(): screen.fill("skyblue") screen.draw.filled_rect( Rect((0, FLOOR_Y), (WIDTH, HEIGHT - FLOOR_Y)), "saddlebrown" ) screen.draw.filled_circle((int(player_x), int(player_y)), 20, "red") hint = "On ground — SPACE to jump" if on_ground else "In the air" screen.draw.text(hint, topleft=(10, 10), fontsize=22, color="white") pgzrun.go()

WALK_SPEED = 4 def update(): global player_x if keyboard.left: player_x -= WALK_SPEED if keyboard.right: player_x += WALK_SPEED player_x = max(20, min(WIDTH - 20, player_x))

# kavitha_jump.py — buggy GRAVITY = 0.5 FLOOR_Y = 360 player_y = 100 vy = 0.0 on_ground = False def update(): global player_y, vy, on_ground player_y += vy vy += GRAVITY if player_y >= FLOOR_Y: vy = 0.0 on_ground = True else: on_ground = False

# kavitha_jump.py — fixed GRAVITY = 0.5 FLOOR_Y = 360 player_y = 100 vy = 0.0 on_ground = False def update(): global player_y, vy, on_ground # Bug 1: gravity must be added BEFORE moving, not after vy += GRAVITY player_y += vy # Bug 2: player_y must be clamped to FLOOR_Y so it doesn't sink through if player_y + 20 >= FLOOR_Y: player_y = FLOOR_Y - 20 vy = 0.0 on_ground = True else: on_ground = False

Learning Goals

Warm-Up · Which Way Is Down?

New Concept · Gravity, Landing, and Jumping

Gravity — three lines

Landing on the floor

Jumping

Worked Example · A Jumping Ball

Try It Yourself

Mini-Challenge · Debug Kavitha's Jump

Recap

Vocabulary Card

Homework

Sample · jump_ceiling.py

Learning Goals

Warm-Up · Which Way Is Down?

New Concept · Gravity, Landing, and Jumping

Gravity — three lines

Landing on the floor

Jumping

Worked Example · A Jumping Ball

Try It Yourself

Mini-Challenge · Debug Kavitha's Jump

Recap

Vocabulary Card

Homework

Sample · jump_ceiling.py

Sample · `jump_ceiling.py`

Sample · `jump_ceiling.py`