Gravity With a Variable — falling that feels real

Learning Goals 3 min

By the end of this lesson you will be able to:

1. Implement gravity in Scratch using a single velocity-y variable that gets more negative by 1 every frame, then is added to the sprite's y.
2. Reset velocity-y to 0 when the player touches a platform colour — so the sprite stops falling instead of sinking through.
3. Tune the feel of gravity by changing one number: change [velocity-y v] by (-1) is normal gravity; change [velocity-y v] by (-2) feels heavy; change [velocity-y v] by (-0.5) feels like the moon.

Warm-Up — drop a coconut 7 min

Watch a coconut fall from a palm tree. The first instant after it lets go, it barely moves. A second later, it's hurtling. By the time it hits the ground, it's fast. It didn't fall at a constant speed — it sped up the whole way down.

Compare this constant-speed Scratch fall:

when flag clicked
go to x: (0) y: (150)
forever
  change y by (-3)
end
        

Try it mentally. The sprite drops smoothly and evenly from the top to the bottom of the Stage. Does that look like a falling coconut, or like a smooth elevator going down?

That variable is velocity-y. Today you'll meet it, and your sprites will finally fall like real things instead of like elevators.

New Concept — velocity-y and acceleration 15 min

Gravity in Scratch is two numbers and one trick.

velocity-y — how fast you're falling right now

Make a new variable, velocity-y, "for this sprite only" on the player. It starts at 0. This variable holds the sprite's current vertical speed. Positive means going up. Negative means going down. Zero means standing still.

Every frame: two changes

Every frame, inside forever, do two things in order:

1. change [velocity-y v] by (-1) — gravity pulls the speed downward.
2. change y by (velocity-y) — apply that speed to the sprite's actual position.

when flag clicked
go to x: (0) y: (150)
set [velocity-y v] to (0)
forever
  change [velocity-y v] by (-1)
  change y by (velocity-y)
end
        

Trace the maths

Walk through what happens after the flag, frame by frame:

• Frame 0: y = 150, velocity-y = 0. Change by −1 → velocity-y = −1. Move y by −1 → y = 149.
• Frame 1: velocity-y = −1. Change by −1 → velocity-y = −2. Move y by −2 → y = 147.
• Frame 2: velocity-y = −2. Change by −1 → velocity-y = −3. Move y by −3 → y = 144.
• Frame 3: velocity-y = −3. Change by −1 → velocity-y = −4. Move y by −4 → y = 140.
• …
• Frame 10: velocity-y ≈ −11. Move y by −11. The sprite is hurtling.

The drops per frame go 1, 2, 3, 4, 5, 6, 7, 8… That's acceleration. That's the coconut. That's real gravity.

Landing — stop falling when you touch a platform

Without landing, the sprite falls forever — straight off the bottom of the Stage. To land, check whether the sprite is touching the platform colour, and if so, reset velocity-y to 0. With velocity-y at 0, the change y by (velocity-y) does nothing — the sprite stops falling.

when flag clicked
go to x: (0) y: (150)
set [velocity-y v] to (0)
forever
  change [velocity-y v] by (-1)
  change y by (velocity-y)
  if <touching color [#964b00] ?> then
    set [velocity-y v] to (0)
    change y by (1)
  end
end
        

Tuning gravity

The −1 in change [velocity-y v] by (-1) is the strength of gravity. Make it more negative for a heavier feel:

• change [velocity-y v] by (-0.5) — moon gravity. The sprite drifts down slowly.
• change [velocity-y v] by (-1) — normal. Earth-like.
• change [velocity-y v] by (-2) — heavy. Like falling through syrup. Or like an anvil.
• change [velocity-y v] by (-4) — slamming. Use for crash-down moves.

One number, the entire physics of your game changes. Mario uses around −1. Sonic uses around −0.7. Crash Bandicoot uses around −1.5.

Worked Example — drop the cat onto a platform 12 min

Open Scratch. Eight steps. We'll build the simplest possible "cat falls onto a floor" demo.

Step 1 — Make the velocity-y variable

Click the cat. Variables panel → Make a Variable → name it velocity-y. Tick "for this sprite only". Tick the show-on-Stage box so you can watch the number change.

Step 2 — Paint a platform sprite

Click the Paint icon to make a new sprite. Use the rectangle tool with brown fill (#964b00). Draw a long, flat rectangle — wider than tall. Name the sprite Floor. Drag it on the Stage to about y: −120.

Step 3 — Position the cat high up

Click the cat. Drag it to x: 0, y: 150 — near the top of the Stage.

Step 4 — Drop in the hat and initial values

when flag clicked
go to x: (0) y: (150)
set [velocity-y v] to (0)
        

Sets the cat back to the top every time you click the flag, and resets the falling speed.

Step 5 — The forever loop

Drag forever under the setup blocks. The next three steps go inside this C.

Step 6 — Gravity (acceleration)

Drag change [velocity-y v] by (-1) inside the forever. This is the gravity strength. Keep it at −1 for now.

Step 7 — Apply the velocity

Drag change y by () below the gravity block. In the empty round slot, drop the round (velocity-y) reporter.

Step 8 — The landing check

Drag if <> then below the change-y. In the diamond: touching color [#964b00]? (click the colour swatch, then click the brown floor on Stage to pick the exact colour). Inside the if: set [velocity-y v] to (0) and change y by (1).

Click the flag

The cat appears at the top. It barely moves for the first half-second. Then it falls faster. Then faster. Then it thunks onto the brown floor and stops dead. Watch velocity-y on the Stage: it ticks down to about −16 by the time the cat lands.

The full assembled stack

when flag clicked
go to x: (0) y: (150)
set [velocity-y v] to (0)
forever
  change [velocity-y v] by (-1)
  change y by (velocity-y)
  if <touching color [#964b00] ?> then
    set [velocity-y v] to (0)
    change y by (1)
  end
end
        

What you just built: the physics engine for every platformer ever made. Two numbers, three lines of code, and your sprite obeys gravity. Add a jump (next lesson) and you've got Mario.

Try It Yourself — three gravity drills 15 min

when flag clicked
go to x: (0) y: (150)
set [velocity-y v] to (0)
forever
  change [velocity-y v] by (-0.5)
  change y by (velocity-y)
  if <touching color [#964b00] ?> then
    set [velocity-y v] to (0)
    change y by (1)
  end
end
          

when flag clicked
go to x: (100) y: (150)
set [velocity-y v] to (0)
forever
  change [velocity-y v] by (-1)
  change y by (velocity-y)
  if <touching color [#964b00] ?> then
    set [velocity-y v] to (0)
    change y by (1)
  end
end
          

when flag clicked
go to x: (0) y: (150)
set [velocity-y v] to (0)
forever
  change [velocity-y v] by (-1)
  if <(velocity-y) < (-12)> then
    set [velocity-y v] to (-12)
  end
  change y by (velocity-y)
  if <touching color [#964b00] ?> then
    set [velocity-y v] to (0)
    change y by (1)
  end
end
          

Mini-Challenge — Arjun's sinking cat 5 min

"My cat falls through the floor and keeps going"

Arjun wrote the gravity stack but his cat sinks straight through the brown floor and disappears off the bottom. Here's his code:

when flag clicked
go to x: (0) y: (150)
set [velocity-y v] to (0)
forever
  change [velocity-y v] by (-1)
  change y by (velocity-y)
  if <touching color [#964b00] ?> then
    change y by (1)
  end
end
        

What's missing inside the if, and why does the cat keep falling without it?

when flag clicked
go to x: (0) y: (150)
set [velocity-y v] to (0)
forever
  change [velocity-y v] by (-1)
  change y by (velocity-y)
  if <touching color [#964b00] ?> then
    set [velocity-y v] to (0)
    change y by (1)
  end
end
          

Recap 3 min

You met the variable trick that powers every platform game in history. Instead of moving y by a fixed amount each frame, you store the falling speed in velocity-y and let gravity change that speed by −1 every frame. The fall accelerates. Add the speed to y, and the sprite falls like a real coconut, not like an elevator. When the sprite touches a platform, reset velocity-y to 0 and nudge up by 1 pixel — that's "landing".

velocity-y

A "for this sprite only" variable that holds the sprite's current vertical speed. Negative = falling. Positive = rising. Zero = not moving vertically.

Acceleration

A change in speed over time. Gravity in Scratch is "speed gets one more negative every frame" — that's constant acceleration downward.

Landing

The moment a falling sprite touches a platform. In code: reset velocity-y to 0 and push the sprite up by 1 pixel so it's no longer overlapping the platform.

Terminal velocity

The maximum falling speed a sprite is allowed to reach. Without a cap, long falls can make velocity-y huge and the sprite teleports through thin platforms.

Gravity strength

The number you subtract from velocity-y each frame. −1 is normal. −0.5 is moon. −2 is heavy. Tuning this one number changes the feel of your whole game.

Homework 2 min

Durian Drop. A durian falls from a tree onto the ground. Same physics as today, fresh skin.

1. Paint a small spiky green durian sprite. Place at x: 0, y: 150.
2. Paint a brown ground (#964b00) at the bottom of the Stage — y around −150.
3. Give the durian a velocity-y variable, "for this sprite only", and tick it on the Stage.
4. Write the gravity loop exactly like today's example. Drop it. The durian should accelerate and thunk onto the ground.
5. Bonus 1: change the gravity strength to −1.5. Does the durian feel heavier?
6. Bonus 2: add a play sound [thud v] inside the if-touching, so it makes a noise on impact.

Save as HW-L3-39-Durian-Drop.sb3.

Bring back next class:

• The .sb3 file.
• Your answer to: "What gravity strength feels most like a real durian to you? Why? (Try −0.5, −1, −1.5, −2 and pick.)"

Heads up for next class: SCR-L03-40 adds jumping. The whole jump is one line: when the up-arrow is pressed and the sprite is on the ground, set [velocity-y v] to (15). Gravity does the rest — the sprite shoots up, slows, peaks, falls back down. Same variable, same loop, jumps for free.