Applying Thrust to an object with Inertia

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I'm using http://www.mathguide.com/lessons2/Vectors.html#combining as a reference. Their magnitude values are too large for my pixels so I'm decreasing by a factor of 100 - so the 30 magnitude becomes 0.3 and the 40 magnitude becomes 0.4.

I have my ship positioned at x = 100, y = 100;
I set its Inertia to equal `theta = 130, magnitude = 0.3`.

Given angle of 0 as being straight up on the screen, the ship moves with inertia towards the bottom right, which is expected.

Everything I have in my `applyThrust` function matches closely with the example code until the very last lines.

I set the new resulting Vector with `theta` & `magnitude` as the Ship's new `Inertia`.

I expect that when the thrust is not active, the Ship continues to move in the bottom right.

Instead, the Ship moves towards the top right - the same angle it moves when thrust is held down.

I think the problem lies in how I'm assigning the new resulting vector to the existing Inertia.

What I'm trying to achieve is:

A ship left alone with no thrust held - continues moving as its `Inertia` dictates.

When Thrust is held, the ship applies thrust towards the theta of its `.r` attribute.

When Thrust is released, the ship continues moving with its new Inertia.

I've uploaded a sample project to Github.

Inertia is defined in engine.js line 257

Code:

   class Inertia extends Component {
    	constructor(o) {
    		super(o);
    		this.mass = o['mass'];
    		this.theta = o['theta'];	
    		this.magnitude = o['magnitude'] ? o['magnitude'] : 0;
    		this.thing = o['thing'];
    	}
    
    	loop() {
    		var thing = this.thing;
    		var origin = {'x' : thing.x, 'y': thing.y};
        var tp = [thing.x, thing.y - 1];
        var np = rotate_point(tp[0], tp[1], origin.x, origin.y, this.theta);
        thing.mx = np.x - thing.x;
        thing.my = np.y - thing.y;
        var p = this.magnitude;
        thing.x = thing.x + thing.mx * p;
        thing.y = thing.y + thing.my * p;
    	}
    }

Apply Thrust:

Code:

	applyThrust() {
		var t1 = this.ship.r; //rotation, the theta the ship is facing
		var m1 = this.ship.grab('thruster').power;

		//Without this if statement, the ship will move towards top right
		//Even when thrust is not active!
		if (m1 > -0.1 && m1 < 0.1) {
			return;
		}
		console.log(m1);
		//m1 is 0 when gamepad button is not active.
		//m1 is 0.4 when the gamepad thrust button is active.

		var inertia = this.ship.grab('inertia');
		var t2 = inertia.theta;
		var m2 = inertia.magnitude;
		
		// console.log(m2);

		//Kim is Inertia
		//Noah is Thrust
		var kimVX = (m2 * Math.cos(t2 * Math.PI / 180));
		var kimVY = (m2 * Math.sin(t2 * Math.PI / 180));

		console.log("Inertia Vector.x" + kimVX);
		console.log("Inertia Vector.y" + kimVY);
		// return;

		var noahVX = (m1 * Math.cos(t1 * Math.PI / 180));
		var noahVY = (m1 * Math.sin(t1 * Math.PI / 180));

		console.log("Thrust Vector.x:" + noahVX);
		console.log("Thrust Vector.y:" + noahVY);
		// return;

		var resultVX = kimVX + noahVX;
		var resultVY = kimVY + noahVY;

		console.log("Resulting Vector.x" + resultVX);
		console.log("Resulting Vector.y" + resultVY);
		// return;

		//So far the console.logs match within 2 decimal places of the example
		//

		var c = resultVX * resultVX + resultVY * resultVY;
		console.log(c); //Expected to equal ~0.3522 since the example is 3522.25
		//and magnitudes are scaled down by a factor of 100
		// return;
		if (c != 0) {


		var resultMagnitude = Math.sqrt(c);
		console.log("Resulting vector magnitude" + resultMagnitude);
		// return;

		var resultTheta = (Math.atan( (resultVY) / (resultVX) )) * 180 / Math.PI ;
		console.log(resultTheta);
		// return;

		inertia.magnitude = resultMagnitude;
		inertia.theta = resultTheta;
		}
	}

The `Ship` is defined in `pilots.js` and is instantiated in `game.js`.
It is represented as a blue circle in `draw.js`.

https://github.com/quantumproducer/sample_vector/

The game can be run from game.html .