If you've spent any time tinkering in Studio, you've probably realized that a roblox custom physics library script can be the difference between a game that feels "okay" and one that feels incredibly polished. Don't get me wrong, the built-in physics engine in Roblox—powered by the Luau VM and the underlying C++ physics solver—is actually pretty impressive. It handles thousands of parts colliding and bouncing around with surprising efficiency. But sometimes, you just don't want "standard" physics. You want something specific, like a character that moves exactly how you want it to, or a car that doesn't fly off into the void because it hit a tiny pebble.
The standard physics engine works on a "rigid body" system. It's meant to be realistic-ish, but for many game genres, realism is the enemy of fun. If you're building a fast-paced platformer or a high-speed racing game, you might find yourself fighting the engine more than using it. That's where writing your own physics logic comes into play.
Why even bother with custom physics?
You might be wondering why anyone would go through the headache of writing a roblox custom physics library script from scratch when Roblox gives you one for free. Well, the main reason is control. The default physics engine can be a bit "jittery" when it comes to high-speed movement. It uses something called a PGS solver, which is great for general purposes but can lead to weird behavior like "ghost collisions" or parts sticking to walls.
Think about a character controller. If you use the default Humanoid object, you're stuck with how Roblox thinks a person should move. It's floaty, the acceleration is sometimes hard to tune, and it's notoriously difficult to make it work on moving platforms. When you build your own library, you decide exactly how friction, gravity, and momentum behave. You aren't asking the engine to calculate these things; you're telling it exactly where a part should be every single frame.
The foundation: Raycasting
At the heart of almost every roblox custom physics library script is raycasting. If you're not familiar, raycasting is basically the script shooting an invisible laser from point A to point B and telling you if it hit anything along the way.
Instead of letting a part fall and wait for the Touched event to fire, you "predict" the movement. Every frame, you look at where the object is going and shoot a raycast in that direction. If the ray hits a wall 2 studs away, and your object was going to move 5 studs this frame, you know you need to stop it at the wall. This approach is much more reliable because it prevents the "tunneling" effect where fast-moving objects clip straight through walls because they were on one side in frame 1 and on the other side in frame 2.
Dealing with the RunService
To make custom physics feel smooth, you can't just use a while true do wait() loop. That's a recipe for stuttering movement. Instead, you need to hook into RunService.Heartbeat or RunService.PreSimulation. These events fire every single time the engine updates, usually 60 to 240 times per second depending on the user's framerate.
When you're writing a roblox custom physics library script, you usually want to handle the logic on the client for the local player and on the server for everything else. However, networking makes this tricky. If you handle physics entirely on the server, players with high ping will feel a delay. If you handle it purely on the client, other players will see that person teleporting around. A good library handles "interpolation" or "client-side prediction" to make things look smooth for everyone, regardless of their internet connection.
Vectors and the math behind the curtain
You don't need to be a math genius to write a physics script, but you do need to get comfortable with Vector3. Everything in custom physics is about vectors. Your velocity is a vector, your position is a vector, and even the direction a surface is facing (the normal) is a vector.
One of the coolest parts of building a library is implementing things like "slopes" or "friction." To handle a character walking up a slope, you take the hit normal from your raycast and use some vector math—specifically the dot product—to figure out how steep the ground is. If it's too steep, you can prevent the player from moving or make them slide down. It's this level of granularity that makes a custom script so powerful.
The nightmare of collisions
Let's be real: writing collision code is the hardest part. Detecting that you hit something is easy (thanks, raycasting!), but deciding what happens after the hit is where it gets messy. This is often called "collision resolution."
If a ball hits a wall, it should bounce. In a roblox custom physics library script, you have to calculate the reflection vector. You take the incoming velocity and "reflect" it across the surface normal. It sounds complicated, but Roblox's Vector3 API actually makes this relatively painless once you get the hang of it. The real struggle is handling "corner cases"—literally. What happens when an object hits two walls at the exact same time in a corner? Without a solid library, your object might get stuck or jitter back and forth.
Optimization: Don't kill the framerate
One thing to keep in mind is that Luau is fast, but it's not infinitely fast. If you try to run complex custom physics for 500 different objects at once on the server, you're going to see some serious lag.
A well-optimized roblox custom physics library script uses tricks to stay efficient. For example, you don't need to run physics calculations for objects that aren't moving. This is called "sleeping." If an object's velocity falls below a certain threshold for a few frames, you just stop calculating its physics until something interacts with it again. Also, using things like WorldRoot:Raycast with a RaycastParams whitelist or blacklist is way faster than just checking every part in the workspace.
Making it modular
If you're going to build a roblox custom physics library script, do yourself a favor and make it modular. Don't just cram everything into one 2,000-line script. Create a ModuleScript for the math utilities, one for the raycast handlers, and another for the actual object states.
This way, when you want to add a new feature—like underwater buoyancy or low-gravity zones—you aren't breaking the entire system. You can just plug in a new module that modifies the velocity before it gets applied to the position. It makes debugging much easier, and believe me, you'll be doing a lot of debugging. There's nothing quite like watching your character turn into a spaghetti monster because of a typo in a vector calculation.
Is it worth the effort?
So, should you actually write a roblox custom physics library script for your next project? If you're making a simple hangout game or a basic "find the markers" style experience, probably not. The default physics are more than enough for that.
But if you're trying to create a unique movement system, a highly competitive shooter with perfectly predictable projectiles, or a platformer that feels like a classic 90s console game, then yes, it's absolutely worth it. It's a steep learning curve, but once you have your own library, you have total freedom. You're no longer playing by Roblox's rules; you're making your own.
The best way to start is small. Don't try to rewrite the whole engine in one day. Start by making a single part move with raycasts instead of velocity. Then try to make it bounce. Then try to make it slide. Before you know it, you'll have a robust system that gives your game a "feel" that nobody else has. And in a sea of millions of Roblox games, having a unique "feel" is one of the best ways to stand out.