Physics and joints

KHIM simulates parts on the game server (and in KHIM Engine's Play). One unit is one stud; gravity is 196.2 units/s², as in Roblox.

Anchored or loose

• An anchored part stays exactly where it is. Floors, walls, buildings.
• A loose part (anchored = false) falls, rolls, is pushed and knocked about. Crates, balls, debris.

local crate = world.Crate
crate.anchored = false
crate:applyImpulse(vec3(0, 500, -300))   -- a shove, divided by its mass
crate.velocity = vec3(0, 50, 0)          -- or set its speed

Materials and mass

A part weighs its volume times its material's density, and its material sets how it grips and bounces:

Material	Density	Friction	Bounce
Plastic, Neon	0.7	0.5	0.1
Smooth	0.7	0.3	0.1
Wood	0.35	0.6	0.05
Glass	2.4	0.25	0.2
Metal	7.8	0.4	0.05
Grass	0.9	0.65	0.05
Brick	1.9	0.75	0.05
Concrete	2.4	0.7	0.03
Sand	1.6	0.85	0
Rock	2.7	0.65	0.05
Water	1.0	0.1	0
Leaves	0.3	0.6	0.02

So the same push moves a big Metal part much less than a small Wood one.

Collision groups

Every part is in one collision group (part.group, "Default" to start). Groups decide what passes through what:

local door = world.VipDoor
door.group = "VipDoor"
physics.collide("VipDoor", "Players", false)   -- players walk through it; crates still bump into it

print(physics.collides("VipDoor", "Players"))  -- false

• Built in: Default, Players (every character) and Ragdoll. A world can name 13 of its own (up to 23 characters each).
• physics.collide(a, b, false) with a == b makes a group's parts pass through each other.
• touched still fires for parts that don't collide, so the VIP door can still tell who went through.

In KHIM Engine you can set the table without code: a part's Inspector > Behavior > CollisionGroup > Which groups collide... (or Insert > Collision groups) - a grid of ticks.

Welds

A weld holds two parts together as one. KHIM Engine keeps welds between parts built together, and a Model's clone() keeps them. Welded parts don't collide with each other.

Joints

A joint goes in the part that moves and names the part it is joined to (part1):

Joint	Does	Like
Hinge	Turns about an axis	A door, a wheel, a lever
Slider	Slides along an axis	A lift, a drawer, a piston
BallJoint	Turns every way about a point	A lamp on a chain
Spring	Pulls toward a rest length	A car's suspension, a trampoline
Rope	Keeps at most a length apart	A swing, a crane's hook

In KHIM Engine: select the part it joins to, Ctrl-click the part that moves, then insert the joint (Insert Object > Joints). The Inspector sets the other part, the anchor, the axis, limits and the motor.

Members

Member
part1	The part it is joined to
anchor, axis	vec3s in its own part's space
enabled	On or off
limited, lower, upper	Hinge: degrees; Slider: units
motor, speed, force	Drive it: degrees or units a second, with up to force
angle (Hinge), offset (Slider)	How far it has turned / slid (read-only)
length, stiffness, damping	Spring and Rope (length for both)

-- A spinning platform on its post (the platform not anchored: joints move only loose parts)
local spinner = new("Hinge", world.Platform)
spinner.part1 = world.Post
spinner.motor = true
spinner.speed = 45        -- degrees a second
spinner.force = 100000

-- A gate made in KHIM Engine: limited 0 to 90, opened by a button
local gate = world.Gate.Hinge
world.Button.Clickable.clicked:connect(function()
    gate.motor = true
    gate.speed = 60       -- swings to its 90-degree stop and holds there
end)

Joints work on the server. A ClientScript can read them but moves nothing with them.

Movers

Put these on a loose part to push it:

Mover	Does	Members
Mover	Drives it at a speed, holding it against gravity too: a moving platform, a drone	velocity, strength, relative
Spinner	Spins it: a fan, a rolling wheel	spin (radians a second about each axis), strength, relative
Thruster	Pushes with a steady force: a rocket, wind	force, relative

local drone = new("Mover", world.Drone)
drone.velocity = vec3(0, 0, -10)   -- flies level at 10 units a second

relative = true makes the vector turn with the part (a Thruster's default: a rocket pushes the way it points).

Moving parts with no physics

For platforms and doors that should follow an exact path, a Motion (no code) or a tween is simpler than physics:

world.Lift:tween({position = world.Lift.position + vec3(0, 20, 0)}, {time = 4, reverses = true, repeats = -1})

Characters standing on a moving part - tweened, on a joint, spinning - ride along with it.

Raycasts

local hit = world:raycast(origin, direction)   -- the first part along the line, or nil
if hit then print(hit.part, hit.position, hit.normal, hit.distance) end

direction's length is how far it looks.