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fix: pendulum in SimplePendulum simulation is rotating inverted #227

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Merged
mattqdev merged 2 commits into from
Mar 9, 2026
Merged

fix: pendulum in SimplePendulum simulation is rotating inverted #227

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130d5dc
fix: pendulum in SimplePendulum simulation is rotating inverted
kakarot2905 Mar 9, 2026
9b910c3
fix: pendulum in SimplePendulum simulation is rotating inverted
kakarot2905 Mar 9, 2026
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32 changes: 18 additions & 14 deletions simulations/SimplePendulum.jsx
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Original file line number Diff line number Diff line change
Expand Up @@ -11,7 +11,11 @@ import {
isPaused,
setPause,
} from "../app/(core)/constants/Time.js";
import { toPixels, toMeters } from "../app/(core)/constants/Utils.js";
import {
toPixels,
toMeters,
physicsYToScreenY,
} from "../app/(core)/constants/Utils.js";

// --- Physics Classes ---
import PhysicsBody from "../app/(core)/physics/PhysicsBody.js";
Expand Down Expand Up @@ -112,8 +116,7 @@ const INPUT_FIELDS = [
];

const SimInfoMapper = (bodyState) => {
const angle =
(Math.atan2(bodyState.position.x, -bodyState.position.y) * 180) / Math.PI;
const angle = (bodyState.angleRad * 180) / Math.PI;

return {
Angle: `${angle.toFixed(1)}Β°`,
Expand Down Expand Up @@ -146,7 +149,7 @@ class PendulumBody extends PhysicsBody {
// Get current angle from vertical
const dx = this.state.position.x - this.anchor.x;
const dy = this.state.position.y - this.anchor.y;
const angle = Math.atan2(dx, dy);
const angle = Math.atan2(dx, -dy);

// Angular acceleration: Ξ± = -(g/L) * sin(ΞΈ) - damping * Ο‰
const angularAcc =
Expand All @@ -160,12 +163,12 @@ class PendulumBody extends PhysicsBody {

// Constrain position to circular path
this.state.position.x = this.anchor.x + this.length * Math.sin(newAngle);
this.state.position.y = this.anchor.y + this.length * Math.cos(newAngle);
this.state.position.y = this.anchor.y - this.length * Math.cos(newAngle);

// Update linear velocity (tangent to circular path)
const speed = this.angularVel * this.length;
this.state.velocity.x = speed * Math.cos(newAngle);
this.state.velocity.y = -speed * Math.sin(newAngle);
this.state.velocity.y = speed * Math.sin(newAngle);

// Update moving state
this.isMoving = Math.abs(this.angularVel) > 0.001;
Expand All @@ -181,7 +184,7 @@ class PendulumBody extends PhysicsBody {
*/
setAngle(angleRad) {
this.state.position.x = this.anchor.x + this.length * Math.sin(angleRad);
this.state.position.y = this.anchor.y + this.length * Math.cos(angleRad);
this.state.position.y = this.anchor.y - this.length * Math.cos(angleRad);
this.angularVel = 0;
this.state.velocity.set(0, 0);
}
Expand All @@ -192,7 +195,7 @@ class PendulumBody extends PhysicsBody {
getAngle() {
const dx = this.state.position.x - this.anchor.x;
const dy = this.state.position.y - this.anchor.y;
return Math.atan2(dx, dy);
return Math.atan2(dx, -dy);
}

/**
Expand Down Expand Up @@ -226,8 +229,8 @@ class PendulumBody extends PhysicsBody {
* Get potential energy (relative to lowest point)
*/
getPotentialEnergy(gravity) {
const lowestY = this.anchor.y + this.length;
return this.params.mass * gravity * (lowestY - this.state.position.y);
const lowestY = this.anchor.y - this.length;
return this.params.mass * gravity * (this.state.position.y - lowestY);
}
}

Expand Down Expand Up @@ -268,14 +271,14 @@ export default function Pendulum() {
// Setup anchor point
anchorRef.current = {
x: toMeters(w / 2),
y: toMeters(h * 0.2),
y: toMeters(h * 0.9),
};

// Calculate initial position
const angleRad = (inputsRef.current.initialAngle * Math.PI) / 180;
const length = inputsRef.current.length;
const initialX = anchorRef.current.x + length * Math.sin(angleRad);
const initialY = anchorRef.current.y + length * Math.cos(angleRad);
const initialY = anchorRef.current.y - length * Math.cos(angleRad);

// Initialize body
if (!bodyRef.current) {
Expand Down Expand Up @@ -375,6 +378,7 @@ export default function Pendulum() {
position: bodyRef.current.state.position,
velocity: bodyRef.current.state.velocity,
angularVel: bodyRef.current.angularVel,
angleRad: bodyRef.current.getAngle(),
kineticEnergy: bodyRef.current.getKineticEnergy(),
potentialEnergy: bodyRef.current.getPotentialEnergy(
inputsRef.current.gravity
Expand Down Expand Up @@ -408,7 +412,7 @@ export default function Pendulum() {
// Draw anchor
const anchorScreen = {
x: toPixels(bodyRef.current.anchor.x),
y: toPixels(bodyRef.current.anchor.y),
y: physicsYToScreenY(bodyRef.current.anchor.y),
};
p.fill(150);
p.noStroke();
Expand All @@ -417,7 +421,7 @@ export default function Pendulum() {
// Draw rope
const bobScreen = {
x: toPixels(bodyRef.current.state.position.x),
y: toPixels(bodyRef.current.state.position.y),
y: physicsYToScreenY(bodyRef.current.state.position.y),
};
p.stroke(inputsRef.current.ropeColor);
p.strokeWeight(2);
Expand Down
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