Getting a custom roblox physics system script to work properly can be a total headache if you don't know exactly where to start, especially when the default engine decides to act a bit wonky. We've all been there—you're trying to build a cool car, a hovering drone, or maybe some weird anti-gravity boots, and suddenly your parts are flying off into the void or jittering like they've had too much caffeine. Roblox's built-in physics engine, powered by Luau and the underlying engine, is actually pretty robust, but sometimes you just need that extra level of control that the standard "drop a part and let it fall" approach doesn't offer.
In this article, we're going to chat about how you can take the reins. We'll look at why you might want your own script, the different ways to manipulate forces, and how to keep things from lagging out your entire server.
Why Even Bother with Custom Physics?
You might be wondering why you'd even bother writing a roblox physics system script when the engine already handles gravity, collisions, and friction. Honestly, the default setup is great for most things, but it can feel a bit "floaty" or "stiff" depending on what you're making.
If you're building a racing game, for example, the default car chassis might not give you that snappy, arcade-like drifting feel you're after. Or maybe you're making a space game where gravity needs to change depending on which planet you're walking on. In those cases, letting the engine do everything automatically just doesn't cut it. You need a script that calculates forces every single frame to make things feel "just right."
The Old vs. The New: Constraints and BodyMovers
Back in the day, we used things like BodyPosition and BodyVelocity. They were simple, but Roblox has mostly moved toward Constraints and newer objects like VectorForce, LinearVelocity, and AngularVelocity.
If you're writing a script nowadays, you're likely going to be toggling these properties or creating them on the fly. The cool thing about these newer objects is that they're much more predictable. They play nicer with the actual physics solver, meaning you won't get as many of those weird "elastic" bounces that used to plague older games.
Using VectorForce for Constant Movement
One of the most common things you'll do in a roblox physics system script is apply a constant force. Think of a jetpack. You don't want it to just teleport the player up; you want it to push them. By using a VectorForce object, you can apply a force relative to the part itself or the world.
If you want a part to hover, you'd calculate the force of gravity (which is mass times workspace gravity) and apply that same amount of force upwards. It's simple math, but it makes a world of difference in how "alive" your objects feel.
Getting into the Scripting Side
When you start writing the actual code, you're going to become best friends with RunService. If you try to run your physics calculations inside a standard while true do wait() end loop, your movement is going to look choppy. It'll stutter, and players will notice it immediately.
Instead, you want to use RunService.Heartbeat or RunService.Stepped. These events fire every time the physics engine updates. This ensures that your custom forces are being applied at the same rate the world is moving.
A Quick Example of Heartbeat
Imagine you're trying to make a part follow a player's mouse, but you want it to have some weight to it. Your roblox physics system script would hook into Heartbeat. Every frame, the script checks where the mouse is, calculates the direction from the part to the mouse, and then applies a LinearVelocity to move it there. Because it's happening 60 (or more) times a second, the motion looks butter-smooth.
Handling Network Ownership
This is the part that trips up almost everyone. If you've ever seen an object in your game stuttering while a player moves it, or if it feels like there's a delay when you try to drive a vehicle, you're dealing with a Network Ownership issue.
Roblox tries to be smart by letting the player's computer handle the physics for things they are touching or controlling. This makes things feel responsive for them. However, if your script is trying to move an object from the server's side while the player also has ownership, you get a "tug-of-war" effect.
In your script, you'll often want to use part:SetNetworkOwner(player) to give control to the client. This makes the physics feel instantaneous for the person interacting with the object, while the server just watches and makes sure nobody is cheating too blatantly.
Making Physics Feel "Juicy"
Let's talk about "juice." This is a game dev term for making things feel satisfying. In a roblox physics system script, juice comes from things like interpolation and damping.
If you move a part from point A to point B instantly, it looks boring. If you use a script to slowly accelerate it, let it overshoot the target slightly, and then bounce back into place (kind of like a spring), it looks professional. Using CFrame:lerp() or TweenService for visual-only physics is one way, but for actual physical objects, you'll want to play with the Damping and Stiffness properties on constraints like SpringConstraint.
The Power of Raycasting
You can't really talk about a custom roblox physics system script without mentioning raycasting. Raycasts are basically invisible lasers you fire out into the world to see what they hit.
If you're making a custom hoverboard, you'd fire a raycast straight down from the board. The script checks how far the ground is. If the ground is 5 studs away, the script applies a lot of upward force. If the ground is 10 studs away, it applies less. This creates a "suspension" effect that feels way better than just setting a fixed height.
Performance Considerations
It's easy to get carried away. You might want to have 500 individual parts all running complex physics calculations at once. Don't do that. Your server will cry, and your players will leave.
When writing your roblox physics system script, always look for ways to optimize. Do you really need to calculate that force every single frame? Maybe you can do it every other frame for objects that are far away. Or better yet, only run the script if a player is within a certain distance of the object.
Roblox is pretty good at "sleeping" parts that aren't moving, so try not to wake them up unless you absolutely have to. If a part is sitting still, let it stay anchored or stationary until someone interacts with it.
Debugging Your Physics
Debugging physics is an experience. Parts will fly away, they'll spin at Mach 5, or they'll just disappear. One of the best tips I can give you is to use the "Show Decomposition Geometry" setting in Studio. This lets you see the actual collision boxes of your objects. Sometimes a part looks like a cube but has a weird, invisible sphere collision box that's messing up your script.
Also, lean heavily on print() statements—or even better, visual debuggers. If your script is calculating a force vector, have it draw a small, bright red line in the direction of that force so you can see what it's doing in real-time. It's a lifesaver when you're trying to figure out why your "hover" script is actually pushing your car into the floor.
Final Thoughts
Building a custom roblox physics system script is one of those things that feels super intimidating at first, but once you get the hang of it, you'll never want to go back to the basic settings. It's all about understanding how forces interact and making sure you're communicating clearly with the engine through RunService.
Whether you're making a complex combat system with knockback, a realistic vehicle, or a wacky physics-based puzzle game, taking control of the physics is the best way to make your game stand out. It just takes a bit of math, a lot of testing, and probably a few instances of parts flying into the stratosphere before you get it perfect. Keep at it, and don't be afraid to experiment with weird values—sometimes the coolest mechanics come from accidental physics bugs!