---
title: "Physics"
url: /docs/en-us/physics
last_updated: 2026-06-24T18:42:15Z
description: "Overview of Roblox's rigid body physics engine, assemblies, and constraints."
---

# Physics

Roblox uses a rigid body physics engine. `Class.BasePart|BaseParts` are subject to physical forces as long as they are not `Class.BasePart.Anchored|Anchored`. You can create physical [assemblies](#assemblies) using attachments and constraints, and you can detect and control [collisions](#collisions) between objects using events and collision filtering.

## Assemblies

An [assembly](/docs/en-us/physics/assemblies.md) is one or more `Class.BasePart|BaseParts` connected by rigid constraints or motors (animated rigid joints). Assemblies can be set to an initial linear or angular velocity, or their behavior can be affected through [constraints](#constraints).

![A light blue cube against a dark blue background that represents an assembly of 1 part.](../assets/physics/assemblies/Assembly-Example-Block.png)_1 assembly; 1 part_

![A humanoid character model against a dark blue background that represents an assembly of 18 parts.](../assets/physics/assemblies/Assembly-Example-Avatar.png)_1 assembly; 18 parts_

![A pirate that represents an assemble of 179 parts.](../assets/physics/assemblies/Assembly-Example-Ship.png)_1 assembly; 179 parts_

## Constraints

Non-anchored assemblies react to force from gravity and collisions, but physical force can also be applied and simulated through [mechanical constraints](/docs/en-us/physics/mechanical-constraints.md) or [mover constraints](/docs/en-us/physics/mover-constraints.md).

#### Mechanical

Mechanical constraints include familiar objects like [hinges](/docs/en-us/physics/constraints/hinge.md), [springs](/docs/en-us/physics/constraints/spring.md), and [ropes](/docs/en-us/physics/constraints/rope.md) which can be used to build mechanisms. Each is summarized in the [mechanical constraints](/docs/en-us/physics/mechanical-constraints.md) guide.

#### Mover

Mover constraints apply velocity, force, or torque to assemblies, align assemblies by position or orientation, and more. Each is summarized in the [mover constraints](/docs/en-us/physics/mover-constraints.md) guide.

## Collisions

Collision events automatically occur when two `Class.BasePart|BaseParts` touch or stop touching in the 3D world. You can [detect](/docs/en-us/workspace/collisions.md) these collisions through the `Class.BasePart.Touched|Touched` and `Class.BasePart.TouchEnded|TouchEnded` events which occur regardless of either part's `Class.BasePart.CanCollide|CanCollide` property value.

Through [collision filtering](/docs/en-us/workspace/collisions.md#collision-filtering) techniques such as collision groups or part‑to‑part filtering, you can control which physical assemblies collide with others.

See the [collisions](/docs/en-us/workspace/collisions.md) guide for more details on detecting and filtering collisions.

## Network ownership

In order to support complex physical mechanisms while also aiming for a smooth and responsive experience for players, the Roblox physics engine utilizes a **distributed physics** system in which computations are distributed between the server and all connected clients. Within this system, the engine assigns **network ownership** of physically simulated `Class.BasePart|BaseParts` to either a client or server to divide the work of calculating physics. See the [network ownership](/docs/en-us/physics/network-ownership.md) guide for further details.

## Adaptive timestepping

The engine emphasizes best performance by automatically assigning assemblies to one of three simulation rates. For scenarios featuring complex mechanisms like tanks, you can improve stability by setting a fixed timestep. See the [adaptive timestepping](/docs/en-us/physics/adaptive-timestepping.md) guide for more information.

## Sleep system

When an assembly is not moving or accelerating, the Roblox physics engine improves performance by automatically stops simulating the assembly. See the [sleep system](/docs/en-us/physics/sleep-system.md) guide for details.