#4. Physics

So far the level is solid and the player walks on it. In this part you add things that move by themselves: crates that fall, stack and get pushed.

#Three kinds of body

Anything with a collider is a physics body. A Rigidbody component decides which kind.

The ground and the steps are static already. The player is a Character Controller, which is a kind of its own.

#Add a crate

1. Press + and Create Cube. Name it Crate 1, and set its Position to -3, 0.5, 2.
2. Add a Box Collider.
3. Add a Rigidbody. Set Type to Dynamic, Mass to 8 and Friction to 0.6.
4. Make a brown material called Wood and put it on the crate.

Duplicate it twice with Ctrl+D and place the copies:

The third sits on top of the other two.

Press Play and walk into the stack. The crates slide and topple. Stop, and they are back where you put them: stopping the game restores the scene exactly as it was when you pressed Play.

#See what physics sees

Press Colliders in the viewport's toolbar. Every collider is outlined: green for static bodies, orange for dynamic ones, cyan for the character.

When something falls through the floor or floats above it, look here first. A box that is drawn but has no outline has no collider.

#Tune it

Select a crate and try the Rigidbody's values. Press Play after each change.

• Mass is in kilograms. The character has a mass too (70 by default), and a heavier crate is harder to push. Try 80.
• Friction is how much it grips: 0 is ice, 1 is rubber on concrete.
• Restitution is bounce: 0 lands dead, 1 bounces back to where it fell from. Lift a crate to height 5 and try 0.8.

#Push from a script

You do not need a script to push crates, the character does that itself. A script can still move bodies when you want it to. This one kicks a crate upwards when the player touches it:

local Bouncy = {}

function Bouncy:OnCollisionEnter(other)
	if other.Tag == "Player" then
		self.Entity.Rigidbody:AddImpulse(Vec3(0, 40, 0))
	end
end

return Bouncy

export = DefineScript({
	OnCollisionEnter(other: Entity) {
		if (other.Tag === "Player") {
			this.Entity.Rigidbody?.AddImpulse(Vec3(0, 40, 0));
		}
	},
});

OnCollisionEnter is called on a script when its entity's body starts touching another. other is the other entity. An impulse is one sudden shove, and AddForce is a steady push for use in OnFixedUpdate.

This script is only an example. The game does not need it.

#What you learned

• A collider gives an entity a shape, and a Rigidbody decides how it moves.
• Stopping the game puts the scene back.
• The Colliders button shows the physics shapes.
• Scripts hear about contacts through OnCollisionEnter.

More in Physics and Rigid bodies and joints.

Next: coins and a score.