Please explain how to add Gravity in Scratch

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Adding gravity is what turns a basic project into a platformer. To make gravity look "real," we can't just move the sprite down at a constant speed. Instead, we need the sprite to accelerate (get faster as it falls).

To do this, we use a Variable.

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Step 1: Create the "Y-Velocity" Variable

A variable is like a container that holds a number. We will use it to track how fast the sprite is currently falling.

1. Go to the Variables category (orange).

2. Click "Make a Variable" and name it y-velocity.

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Step 2: The Core Gravity Script

This script does two things: it pulls the sprite down and stops it when it hits the ground.

The Logic:

1. When 🚩 clicked

2. set [y-velocity] to 0 (Reset speed at start)

3. forever

   • change [y-velocity] by (-1) — This is the "Gravity" constant. It makes the downward speed faster every second.

   • change y by (y-velocity) — This actually moves the sprite.

   • if <touching [Ground Sprite or Color?]> then

     • set [y-velocity] to 0 — This stops the falling.

     • repeat until <not <touching [Ground]?>>

       • change y by (1) — This prevents the sprite from getting stuck inside the floor.

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Step 3: Adding the Jump

Now that gravity is pulling you down, you need a way to go up!

Inside your forever loop (and inside the if touching ground block), add this:

• if <key [space] pressed?> then

  • set [y-velocity] to 15

Why this works:

Because you set the y-velocity to a positive number ($15$), the sprite will suddenly shoot upward. Then, because the gravity script is always running, it will slowly subtract $1$ from that $15$ until it becomes $0$ (the peak of the jump) and then becomes negative (falling back down).

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Summary of the Physics

• Gravity: change [y-velocity] by (-1)

• Movement: change y by (y-velocity)

• Ground: set [y-velocity] to 0

• Jumping: set [y-velocity] to 15

Tip: If you want "Moon Gravity," change the -1 to -0.2. If you want "Jupiter Gravity," change it to -5!

Would you like me to explain how to make a "Scrolling Background" so your character can travel through a long level?

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