If you've ever felt like the standard engine is fighting against your game design, writing a roblox custom physics script is usually the logical next step to gaining total control. Let's be real for a second: the built-in Roblox physics engine is actually incredible for what it does. It handles thousands of parts colliding and bouncing around with pretty decent performance. But sometimes, it's just too bouncy. Or too floaty. Or you're trying to make a character walk on the ceiling, and the engine starts screaming at you because it thinks "down" should always be toward the bottom of the map.
That's where the power of scripting comes in. When you step away from the default behavior and start defining your own rules, you aren't just a builder anymore; you're basically the god of your own little digital universe.
Why Even Bother With Custom Physics?
You might be wondering why anyone would put themselves through the headache of math and vectors when Roblox provides BodyMovers (now mostly replaced by constraints) right out of the box. The truth is, the default physics engine is a generalist. It's designed to handle everything from a falling brick to a complex ragdoll. Because it's a generalist, it doesn't always excel at specific, "gamey" mechanics.
Think about arcade racing games. If you used pure, realistic physics for a high-speed racer, the car would flip over the moment you hit a pebble at 200 mph. It would be frustrating and unplayable. By using a roblox custom physics script, you can tell the game, "Hey, I know gravity says this car should fly off the track, but I want it to stick to the road like glue." It's about making the game feel right, rather than making it scientifically accurate.
The Foundation: RunService and Raycasting
To get started with any custom movement system, you need to understand the heartbeat of the game. You can't just use a while true do wait() loop. That's way too slow and jittery. Instead, you're going to be living in RunService.Heartbeat or RunService.RenderStepped.
Heartbeat fires every single frame after the physics simulation has happened. This is where you'll do most of your heavy lifting. If you're trying to move a part manually, you'll calculate the new position in this loop and update the CFrame.
But how does your script know where the "ground" is if you aren't using the default collisions? That's where Raycasting comes in. Think of Raycasting as an invisible laser beam you fire from a part. You tell the script: "Shoot a laser five feet straight down. If it hits something, tell me how far away it is." If the laser says the ground is only three feet away, your script knows it needs to push the object up to keep it hovering at five feet. This is the basic logic behind almost every hovercar or custom character controller on the platform.
Breaking Gravity's Rules
One of the coolest things you can do with a roblox custom physics script is messing with gravity. We've all seen those games where you can walk on walls or jump between small planets with their own gravitational pulls. You simply cannot do that with the default settings.
To make this work, you usually have to disable the "falling" state of a character or set the gravity of the entire workspace to zero, and then manually apply a downward force yourself. By calculating the vector between the player and the center of a planet, you can apply a constant force that pulls them toward that planet. It sounds complicated, but once you see the math click, it feels like magic. You're no longer bound by the Y-axis. Up, down, left, right—these become whatever you want them to be.
Handling Collisions Without Getting Stuck
The hardest part about writing your own physics logic is handling collisions. When you use the default engine, parts just stop when they hit a wall. When you're manually setting the CFrame of a part every frame, you might accidentally teleport that part inside a wall.
To fix this, you have to get a bit clever with your math. Most developers use a "sphere-cast" or multiple rays to check the surrounding area before moving. If the script detects that the next move will put the object inside a wall, it calculates a "slide" vector. Instead of just stopping, the object slides along the surface of the wall. This is what makes movement feel smooth and professional rather than clunky and buggy.
Performance: The Silent Killer
Here is the thing about a roblox custom physics script: it can be a total resource hog if you aren't careful. If you have one object running a complex script, the server won't even notice. If you have 100 objects all firing ten rays per frame and doing complex vector math, your server's heart rate is going to spike, and your players are going to experience that dreaded "ping lag."
To keep things optimized, you should always try to handle physics on the Client (the player's computer) whenever possible. For example, if you're making a custom car, let the person driving the car calculate the physics. Then, just tell the server where the car is so other players can see it. This is called "Network Ownership," and it's the secret sauce to making high-performance Roblox games.
Also, don't calculate things you don't need. If an object isn't moving, don't run the physics script on it! Put it to "sleep" until something interacts with it.
Making it Feel "Juicy"
Once you have the basic movement down, you need to add the polish. This is the difference between a blocky, robotic-moving part and something that feels alive. You can use Lerping (Linear Interpolation) or Tweens to smooth out the transitions.
If your custom script moves a character, don't just snap their orientation to the direction they're walking. Use a bit of math to rotate them toward that direction over a few frames. It adds a sense of weight and momentum. Physics is as much about art as it is about math. You want the player to feel the weight of a heavy boulder or the nimbleness of a light aircraft.
Where to Start Learning?
If you're sitting there thinking, "I don't even know how to code a raycast," don't sweat it. Nobody starts out knowing this stuff. The Roblox Developer Hub (now documentation site) is actually really good, and there are countless open-source modules created by the community.
Look into things like the "Chickynoid" project if you want to see how top-tier developers handle custom character physics with server-side networking. Or just start small. Try to make a part that hovers exactly four studs above whatever surface is below it. Once you get that working, try to make it move forward without clipping through walls.
Final Thoughts
Creating a roblox custom physics script is definitely a challenge, but it's one of the most rewarding skills you can pick up as a scripter. It moves you away from just "using" the engine and toward "directing" it. Whether you're building a realistic flight sim, a gravity-defying platformer, or just a weird bouncy ball that defies the laws of nature, custom physics gives you the keys to the kingdom.
It's going to involve some trial and error. You're going to see parts fly off into the void at Mach 5 because you accidentally multiplied a number instead of dividing it. You're going to get frustrated when your character gets stuck in a floor. But when you finally stick that landing and the movement feels exactly how you imagined it in your head? There's no better feeling in game dev. So, fire up Studio, open a fresh script, and start messing with those vectors!