Table of Contents
Understanding Advanced Physics in Roblox
Advanced physics systems in Roblox let you move from simple falling parts and basic collisions to games that feel physical, believable, and expressive. In this chapter you will focus on how to control and customize physics so that your game can have vehicles, ropes, sliding objects, floating platforms, and many other interactive systems.
Roblox already simulates gravity, collisions, and movement. Advanced physics means shaping and controlling that simulation with constraints, custom forces, and server friendly setups. You are not replacing the physics engine, you are guiding it.
Physics Simulation Basics You Must Respect
Before building complex systems you need to work with the rules of Roblox physics, not against them. Every physical object in Roblox is a BasePart. Each part has properties such as Anchored, CanCollide, Mass, CustomPhysicalProperties, and AssemblyLinearVelocity. A group of parts welded or constrained together behaves as a single assembly. Roblox solves movement per assembly, not per individual part inside it.
If you want something to simulate realistically, it must not be anchored. An anchored part ignores forces, gravity, and velocity. Joints and constraints also become meaningless if both sides are anchored. When you create cars or mechanisms, usually the main body is unanchored and the model is put together with welds and constraints.
You can inspect how physics behaves by using the “Show Decomposition Geometry” and “Show Contacts” options in the Studio settings. These views help you see how Roblox approximates shapes for collision. Complex meshes are often simplified internally. This is important when you try to build stable contraptions, because unstable contact shapes can cause jitter or parts that bounce unpredictably.
Roblox uses a fixed time step internally, so your physics will be consistent across different frame rates. However, if you apply forces or set velocities from scripts you should always think in units per second and per second squared. A velocity of $(10, 0, 0)$ in AssemblyLinearVelocity means 10 studs per second along the X axis. Gravity by default is workspace.Gravity in studs per second squared, with a default value of 196.2, which approximates $9.81$ meters per second squared with a scale conversion.
Important physics rule: Only unanchored assemblies react to gravity, forces, and constraints. Anchored parts ignore physics and can destabilize constraints.
Controlling Motion with Body Movers and Forces
To create advanced motion you rarely move parts directly with CFrame inside the main game, because that overrides the physics engine. Instead, you apply forces and let the engine solve how the part moves. Roblox has several classes for this purpose.
Historically, there were “Body movers” like BodyForce, BodyVelocity, and BodyPosition. These are now considered legacy and are being replaced by newer classes such as VectorForce, LinearVelocity, AlignPosition, and AlignOrientation. You should use the newer objects for new projects.
A VectorForce applies a continuous force on an assembly. It has a Force vector measured in Newton like units that Roblox uses internally. By Newton’s second law, force $F$ equals mass $m$ times acceleration $a$, written as
$$F = m \cdot a.$$
If you want to accelerate an object with mass $m$ by a specific acceleration $a$, you compute the force vector as $F = m \cdot a$. Roblox can also automatically scale for mass if you use the RelativeTo and ApplyAtCenterOfMass properties correctly.
LinearVelocity is a constraint that directly controls the velocity of an assembly rather than the force. It tries to make the assembly match a target VectorVelocity. This can be useful for hovercraft, moving platforms, and custom character controllers, because you specify the desired speed and Roblox computes the required forces.
AlignPosition and AlignOrientation are constraint like objects that try to keep one part aligned in position or rotation with a target. They are flexible and can create smooth motion when combined with appropriate responsiveness and max force or torque limits.
When you drive motion through these components, your scripts configure the constraints and forces, then the physics engine does the heavy lifting. This avoids teleporting, reduces desync issues, and makes your systems interactable by other forces such as explosions, collisions, and other constraints.
Key practice: Prefer VectorForce, LinearVelocity, AlignPosition, and AlignOrientation instead of directly setting CFrame every frame. Let the physics engine solve movement whenever possible.
Building with Constraints and Joints
Constraints are at the heart of advanced physics systems in Roblox. They connect parts and define how they can move relative to each other. Classic joints such as Weld and Motor6D essentially glue parts together and prevent relative motion. Constraints, in contrast, allow controlled motion such as rotation, sliding, or limited freedom.
A HingeConstraint lets one part rotate around a single axis relative to another. This is ideal for doors, wheels, and simple levers. It has properties like ActuatorType, MotorMaxTorque, and AngularVelocity. When you set ActuatorType to Motor the hinge can drive itself, which is how you can power wheels or rotating platforms.
BallSocketConstraint allows rotation around any axis, like a universal joint in real life. It is often used for ragdolls and flexible connections where you do not want the rotation limited to one axis. You can still limit the range using angular limits to prevent unrealistic twisting.
PrismaticConstraint lets one part slide along a line relative to another. This is used for pistons, elevators, and sliding doors. Like the hinge, it can have a motor like behavior that lets it extend or retract automatically.
There are additional constraints like SpringConstraint, which connects two attachments with a spring like behavior, and RodConstraint, which tries to keep two points at a fixed distance. These are useful for suspension, ropes, and dynamic bridges.
All constraints connect Attachment objects which you place inside parts. An attachment stores a position and orientation relative to its parent part. By editing attachments you precisely control how parts link together. Typically, you create two attachments, one in each part, then create a constraint that references those two attachments.
When building mechanical systems you should keep assemblies clean and avoid mixing anchored parts inside a constrained structure. Use either constraints or welds as your main structure, not a confusing mix of both. This reduces instability and makes it easier to predict how your system will behave.
Ragdolls and Character Physics
Ragdolls are a classic use of advanced physics. A ragdoll character replaces some or all of the standard character joints with physically driven constraints such as BallSocketConstraint. Instead of animations controlling the limbs, the physics engine does, and external forces such as explosions or collisions make the body flop realistically.
To create a ragdoll you usually begin with a normal Roblox character rig which has Motor6D joints between the body parts. A script converts each motor into a constraint when the character dies or enters ragdoll mode. The script creates matching attachments on each side of the joint, then creates a BallSocketConstraint or a HingeConstraint with suitable angular limits to approximate real joint motion.
It is important to tune the damping and limits of each constraint. Too much freedom and the limbs will twist in impossible directions. Too tight and the ragdoll will look stiff. You also want to consider the character’s mass distribution. If certain parts are too heavy relative to others, the ragdoll may behave oddly or collapse in strange ways.
Ragdolls also need to be kept server friendly. The ragdoll behavior itself should run on the server to keep all players in sync. However, you might choose to use client side effects, such as camera shake or screen blur, around the ragdolled character.
A partial ragdoll is also possible. For example, you can leave the torso controlled by animations but ragdoll only the arms when the player is stunned. In this case your script only converts the relevant joints to constraints for a short time, then restores the original Motor6D when the effect ends.
Vehicles and Wheeled Systems
Vehicles are one of the most common reasons to build advanced physics systems. A simple approach is to move a car model by setting its CFrame, but this ignores proper friction, suspension, and collisions. A physically simulated vehicle uses constraints for wheels, suspension, and sometimes steering.
The core of a physics based car uses HingeConstraint for each wheel. The hinge axis runs through the wheel so it can rotate. If the hinge ActuatorType is set to Motor, you can set AngularVelocity to spin the wheel, and Roblox will apply torque to actually move the vehicle. The ground friction then pushes the car forward.
Suspension can be built two different ways. One approach uses SpringConstraint objects that connect the car body to the wheel hubs. The springs compress when the car goes over bumps and help keep the wheels in contact with the ground. Another approach uses custom arrangements of PrismaticConstraint to control how far the wheel can travel vertically.
Steering usually involves rotating the front wheels around a vertical axis. You can mount the wheel on a part that can rotate left and right around a vertical hinge. When the player presses left or right your script adjusts the constraint angle or control target, which turns the wheels, and friction then pulls the car toward the turn.
When tuning vehicles, pay special attention to friction and weight. You can adjust Friction and Elasticity using CustomPhysicalProperties on the wheel parts and ground. Too little friction and the car slides everywhere. Too much and it may roll over easily or feel glued to the track. Adjust the total mass of the car so that gravity and suspension work in a reasonable range, and avoid extremely large or tiny cars that push the engine beyond its comfortable limits.
Ropes, Chains, and Soft Systems
Ropes, cables, and chains create a lot of visual interest and can make environments feel dynamic. In Roblox, ropes are usually built with constraints rather than real flexible meshes. A simple rope is a sequence of small parts linked by BallSocketConstraint or RodConstraint. Each link can move, and together they approximate a rope or chain.
For a simpler setup you can use RopeConstraint which connects two attachments and simulates a rope between them. The visual rope is drawn by Roblox, and the physics treats it like a flexible connection that can slacken and become taut. You can control properties like length, rest length, thickness, and whether it can stretch.
Although these systems look soft, they are still based on rigid parts and constraints. They can become unstable or swing wildly if you build them too long or too heavy. To keep them stable, use relatively short segments, limit the total length, and avoid extremely massive parts hanging on the rope unless you have very strong constraints.
You can combine ropes with SpringConstraint to build bouncy bridges or hanging platforms. In this arrangement, each anchor point of the platform is connected with a spring or rope to the ceiling. When a player walks on the platform it bounces and sways. Correct tuning of stiffness and damping is important so the platform does not oscillate forever or shake itself apart.
For more advanced soft behavior such as cloth or large deformable shapes, Roblox does not provide full soft body physics. Instead you approximate softness with small rigid pieces and constraints, or you fake the effect visually with animations and particle effects, while keeping the physical collision shape simpler.
Custom Gravity and Planetary Worlds
Not every game needs normal downward gravity. In Roblox you can create spaces where gravity points in different directions or has different strengths. The simplest technique is to change workspace.Gravity. This affects the entire world equally, so it is suitable for low gravity maps or underwater like settings.
For localized or directional gravity you use forces on specific parts or characters. One common trick is to set workspace.Gravity to zero and then apply your own force to each affected assembly using VectorForce or LinearVelocity. For a planetary world, you compute the direction from the character toward the planet center, then apply a force in that direction.
If the planet center is at position $C$ and the character root position is $P$, then the direction from the character to the center is
$$\hat{d} = \frac{C - P}{\lVert C - P \rVert}.$$
You can then compute a gravity acceleration of magnitude $g$ as
$$a = g \cdot \hat{d},$$
and the force to apply to an assembly of mass $m$ is
$$F = m \cdot a.$$
By updating this force each frame or on a frequent interval, you make the player fall toward the center of the planet from any side.
For a full planetary system you must also adjust character orientation. The character’s “up” direction should always point away from the planet center. You can use AlignOrientation to match the character’s up vector to the negative of the gravity direction. This avoids the squashed look you would get from simply rotating the HumanoidRootPart with CFrame every frame.
Custom gravity setups are more complex than standard gravity and can be heavier on performance. You should limit them to the assemblies that actually need them, and avoid very frequent expensive computations for every minor part.
Stability and Performance Considerations
Advanced physics systems can be fragile if you do not respect stability and performance. Every constraint, movable part, and contact adds work to the physics engine. Hundreds of fully simulated cars, ropes, and ragdolls at the same time can quickly become too heavy.
To keep physics stable, avoid very extreme values. Unrealistically high forces, velocities, or very small parts can cause jitter and explosions of energy. Try to keep object sizes in a reasonable range, for example human sized or vehicle sized, and avoid parts with dimensions that are extremely small compared to others. Values like velocities of hundreds of studs per second can create tunneling where parts pass through each other before the engine detects the collision.
If you need performance, consider sleeping or disabling systems that are not in use. A parked car that no player is touching can be anchored or simplified until someone gets near it. A ragdoll that has finished falling can be converted back to a simple static pose. Long ropes that are far from any player might be hidden or replaced with a static mesh.
The network model is also important. Physics is simulated on the server, with clients receiving updates. Roblox also uses Network Ownership, where the simulation authority for certain assemblies can be delegated to clients. Too many physics heavy objects with ownership on different clients can lead to desyncs. As a beginner, you can generally let Roblox handle ownership automatically, but you should avoid applying conflicting forces from both client and server on the same assembly.
Scripted physics that fights the engine is another common source of trouble. For example, if every frame a script sets a part’s CFrame while a constraint tries to move it differently, the result will be jitter, popping, or strange delays. In advanced systems, your scripts should mostly adjust constraint targets and forces instead of overriding the resulting positions.
Designing Physics Systems for Gameplay
Advanced physics is not just for realism, it is a gameplay tool. Before you build a complex system, always ask what purpose it serves in your game. A car that feels slightly arcadey but is easy to control may be better than a perfectly realistic car that is frustrating. A rope bridge that sways just enough to be fun is better than an ultra realistic one that causes motion sickness.
Use physics as a way to create interesting decisions. Swinging platforms can require timing. Rolling boulders can force players to dodge. Weight sensitive switches can ask players to stack objects. Vehicle physics can create skill based race tracks. Ragdolls can sell the impact of combat in an arena.
You also need to think about predictability. Random physics outcomes can be funny but also unfair. If a bridge sometimes throws a player into the air for no obvious reason, players will blame the game rather than themselves. Test your systems thoroughly and adjust stiffness, damping, mass, and constraints so that behavior is consistent.
Finally, build your advanced physics pieces as reusable modules. A well tuned car, rope, or door mechanism can be copied into future games. Experienced Roblox developers build libraries of reliable physics prefabs that they can plug into new projects without rebuilding everything from scratch.