Getting a roblox custom physics script to behave exactly how you want is probably one of the most satisfying parts of game dev on the platform. We've all been there: you're trying to make a vehicle, a floating platform, or some weird gravity-defying gadget, and the default Roblox physics just aren't cutting it. Maybe the parts are bouncing too much, or maybe they feel like they're made of lead when they should feel like feathers. That's when you realize you need to take the reins and write something yourself.
The default physics engine in Roblox—based on the high-performance "PGS" solver—is actually incredibly powerful, but it's designed to be a general-purpose tool. It handles collisions and gravity for millions of games. However, when you want that specific "snappy" feeling of a platformer or the drift of a stylized racing game, a generic solver won't give you that level of control. You need a script that tells the engine exactly how to move things, frame by frame.
Why go custom in the first place?
You might wonder why you'd bother writing a roblox custom physics script when you could just tweak the properties in the Properties window. The truth is, while you can change density, friction, and elasticity, those are still global rules. If you're building something like a hoverboard, you don't necessarily want it to "collide" with the ground in a traditional sense; you want it to maintain a specific distance from the surface using a constant upward force.
When you write your own physics logic, you stop fighting the engine and start guiding it. You can create systems that ignore gravity for specific objects, or objects that get pulled toward a moving point rather than just falling down. This is how the "big" games on Roblox manage to have such unique movement mechanics. They aren't just letting the engine do the work; they're calculating positions and velocities manually using Luau.
The heartbeat of physics: RunService
If you're going to dive into a roblox custom physics script, your best friend is going to be RunService. Specifically, you'll be looking at Heartbeat or Stepped. If you try to run physics logic inside a standard while wait() do loop, your game is going to look jittery and unprofessional.
Heartbeat fires every single frame after the physics simulation has completed. This is the perfect spot to apply your custom forces. Because it runs so frequently, any movement you apply will look smooth to the player. If you're doing something that needs to happen before the physics engine solves collisions, you'd use Stepped. Most of the time, for things like custom gravity or hovering, Heartbeat is the way to go.
Forces vs. CFrame: Choosing your weapon
There are generally two ways to handle a roblox custom physics script. You can either use Forces (like VectorForce, LinearVelocity, or the older BodyMoving objects) or you can use CFrame.
Using Forces is usually the "safer" way. You're still letting the physics engine handle the actual movement and collisions, but you're just giving it a nudge. For instance, if you want a part to float, you apply a VectorForce that points upward with the exact same magnitude as gravity. It's clean, it respects walls, and it doesn't cause parts to clip through the floor.
On the other hand, manipulating the CFrame directly is like being a god. You are telling the part exactly where to be every frame. This is great for things like moving platforms or cinematic camera tracks, but it can be a nightmare for collisions. If you CFrame a part directly into a wall, it won't stop—it'll just teleport right through it. If you go this route, you usually have to write your own collision detection using raycasting, which is a whole different rabbit hole.
The importance of network ownership
One thing that trips up almost everyone making a roblox custom physics script for the first time is Network Ownership. Have you ever made a cool physics-based car, and it moves perfectly when you test it alone, but as soon as a friend joins, it starts lagging and stuttering? That's a network ownership issue.
Roblox tries to be smart by letting the player's computer calculate the physics for their own character and anything they're "driving." If the server is trying to run a physics script on a part that the client owns, you get a tug-of-war. The server says "be here," and the client says "no, I'm here." To fix this, you have to use SetNetworkOwner(nil) to force the server to handle it, or pass ownership to the player if they're the ones controlling the object. It's a small step that saves hours of hair-pulling.
Making things feel "Juicy"
A great roblox custom physics script isn't just about moving a block from point A to point B. It's about the way it moves. This is where math becomes your secret weapon. Using things like spring modules or lerping (Linear Interpolation) can make your physics feel alive.
Imagine a floating crystal. If it just sits there, it's boring. But if you use a sine wave in your script to make it bob up and down, and maybe a little bit of lerping to make it tilt when it moves, it suddenly feels like a polished part of a professional game. You can find plenty of spring scripts in the developer community, and plugging them into your physics logic is a total game-changer. It adds that "bounce" and "weight" that players subconsciously associate with high-quality games.
Raycasting: The eyes of your script
You can't really talk about a roblox custom physics script without mentioning raycasting. Raycasting is how your script "sees" the world. If you're making a hovercar, you cast a ray downward from the bottom of the car. The ray tells the script how far away the ground is. If the ground is 5 studs away, you apply a little bit of force. If it's 2 studs away, you apply a lot of force to push it back up.
This feedback loop—detecting the environment and then reacting with a force—is the core of almost every advanced physics system on the platform. It's how character controllers know if they're on the ground or in the air, and how homing missiles know where their target is.
Performance considerations
It's easy to get carried away and start putting a roblox custom physics script on every single object in your game. However, you have to remember that scripts take up CPU cycles. If you have 500 parts all running complex math on every Heartbeat frame, the frame rate is going to tank, especially for players on mobile or older laptops.
The trick is to be efficient. Don't run the script if the object is too far away from any players. Use "sleep" logic—if the part hasn't moved in a while and nothing is touching it, disable the script until something wakes it up. Optimization isn't the most fun part of coding, but it's what makes your game playable for everyone, not just people with high-end gaming rigs.
Common mistakes to avoid
One of the biggest blunders I see is people trying to use wait() inside their physics calculations. Just don't do it. Physics needs to be consistent. Even a task.wait() is too slow and unpredictable for high-speed physics. Stick to the delta time (dt) provided by the Heartbeat event. This dt value tells you exactly how much time passed since the last frame, allowing you to adjust your movement so it's consistent regardless of whether a player is running at 30 FPS or 144 FPS.
Another pitfall is ignoring the "Mass" of the parts. If your script applies a fixed force of 500, it might move a small brick way too fast but barely nudge a giant boulder. Using Part:GetMass() in your calculations ensures that your roblox custom physics script works uniformly across objects of different sizes.
Wrapping it up
At the end of the day, creating a roblox custom physics script is all about experimentation. You're going to have parts flying off into the void, objects spinning uncontrollably like Beyblades, and plenty of "why is it doing that?" moments. But that's the process.
Once you get that first hovercraft to stay level, or that first custom character to jump exactly the way you imagined, you'll realize how much more freedom you have. You aren't just playing within the boundaries Roblox set; you're making your own rules for how the world works. So, open up Studio, create a script, hook it up to RunService, and start messing around with some vectors. You might be surprised at what you can build when you stop relying on the default settings.