Lesson 10 of 22, in world 3: Physics & Collisions

Gravity, Jumping & Collision Response

Time: About 25 min
Builds on: Collision Detection Velocity, Acceleration & Friction Handling Input Delta Time & Fixed Timesteps

Detecting overlap tells you that you hit a wall. Collision response decides what happens next — usually, push the object back out so it’s touching but not inside. Combine that with gravity and a jump and you have the heart of every platformer.

Gravity is just acceleration

From lesson 6: acceleration changes velocity. Gravity is a constant downward acceleration, so every frame vy += GRAVITY * dt. A jump is a single instant upward kick: vy = -JUMP_SPEED. Gravity then slows the rise, stops it, and pulls you back down — tracing a parabola.

LiveA tiny platformer

Gravity 1800 px/s²

Jump speed 620 px/s

Coyote time (0.1 s)Variable jump heightShow collision resolution

Move with ←→, jump with Space or ↑. Tap jump for a short hop, hold it for a high jump. Walk off a ledge and jump a moment late — coyote time still lets you. The dotted trail is your jump arc: a parabola, because gravity is a constant acceleration.

Resolving collisions one axis at a time

If you move diagonally into a corner, which way should you be pushed out? The simplest robust approach: move along x, fix any overlap along x; then move along y, fix any overlap along y. Because only one axis changed each time, you always know which side you came from.

JavaScript

// Horizontal
player.x += player.vx * dt
for (const wall of solids) if (overlaps(player, wall)) {
  player.x = player.vx > 0 ? wall.x - player.w   // moving right → snap to wall's left side
                           : wall.x + wall.w     // moving left  → snap to its right side
  player.vx = 0
}
// Vertical
player.y += player.vy * dt
player.onGround = false
for (const wall of solids) if (overlaps(player, wall)) {
  if (player.vy > 0) { player.y = wall.y - player.h; player.onGround = true }  // landed
  else player.y = wall.y + wall.h                                              // bonked head
  player.vy = 0
}

Game-feel tricks every platformer uses

Coyote time — allow a jump for ~0.1 s after walking off a ledge (like Wile E. Coyote hanging in mid-air). Players feel they pressed in time, because they almost did.
Jump buffering — from the input lesson: remember an early jump press until you land.
Variable jump height — if the jump button is released while still rising, cut the upward velocity. Tap = hop, hold = high jump.
Higher gravity when falling — makes jumps feel snappy instead of floaty.

In Godot

CharacterBody2D.move_and_slide() does the move-and-resolve step for you (including slopes), and is_on_floor() tells you if you landed. You still write gravity, jumping, coyote time and buffering yourself:

GDScript

func _physics_process(delta):
    if not is_on_floor():
        velocity.y += gravity * delta
    if Input.is_action_just_pressed("jump") and is_on_floor():
        velocity.y = -jump_speed
    if Input.is_action_just_released("jump") and velocity.y < 0:
        velocity.y *= 0.45
    move_and_slide()

Key takeaways

Gravity: vy += g·dt every frame. Jump: set vy to a negative value once.
Resolve collisions one axis at a time: move x → fix x, move y → fix y.
Coyote time, jump buffering and variable jump height make controls feel fair.

Check yourself

Q1. After landing on a platform, what should vy become?

Q2. Why resolve x and y separately?

Q3. A player runs off a cliff and presses jump 3 frames later. It works. Why?

Exercises

Turn coyote time off and try to jump at the very edge of the first gap. How often do you fail?
Add "fall gravity": use 1.6× gravity when vy > 0. Does the jump feel better?
Add a double jump: a counter that resets on landing and allows one jump in mid-air.