# mathematics – Predict future position of a moving body in Phaser arcade physics

I am looking for an equation for predicting the future position of a moving arcade physics body in Phaser 3. The body has drag applied and isDamping set to true. Phaser applies the drag using the following run on each update (showing x axis only but same applies to y).

``````//  Damping based deceleration
dragX = Math.pow(dragX, delta);

velocityX *= dragX;
``````

Given the above, how would I write a kinetic equation to predict the future position?

I am currently using the method below, where I am iterating over frames to calculate the position accumulatively. But this is inefficient and inelegant so would prefer a solution which estimates the position without any looping.

``````public futurePosition(timeInSeconds: number): Phaser.Math.Vector2 {
const DELTA = 1 / 60; // Assume we are running at 60fps
const DRAG = 0.3; // Drag value
const position = this.position.clone(); // Current position of body
const velocity = this.body.velocity.clone(); // Current velocity

// Inefficiently looping through frames
for (let i = 0; i < timeInSeconds / (DELTA * 1000); i++) {
velocity.x *= Math.pow(DRAG, DELTA);
velocity.y *= Math.pow(DRAG, DELTA);
position.x += velocity.x * DELTA;
position.y += velocity.y * DELTA;
}

return position;
}
``````

Any help appreciated. Thanks.