Mysterious factor of 150 on SpriteKit's Physics. Gravity and Forces

Question

I wanted an object to float on the screen, resisting gravity, not moving at all.

This is the gravity setting of the view.

    self.physicsWorld.gravity = CGVector(dx: 0, dy: 5.0)

it's set to 5m/s^2 upwards. So object gets accelerated by 5m upwards per second.

Mass of the object is set to 1.0kg

    self.physicsBody?.mass = 1.0

I applied a force to the object so it can resist the gravity. So I did the following.

func update(delta: TimeInterval) {
    ...

    let force = CGVector(dx: 0.0, dy: -5.0)
    self.physicsBody?.applyForce(force)
}

I applied -5N because I thought the gravitational force applied to the object is 1kg * 5m/s^2 = 5N. Applying -5N will make the object gets accelerated by -5m/s^2, floating on the screen as a result with the gravity.

But it did not work. Instead I had to do this.

    let force = CGVector(dx: 0.0, dy: -5.0 * 150.0)

-5 multiplied by 150 is -750. So, where does this 150 come from? Why do I have to apply -750N instead of -5N to make the object resist gravity?

I also tested out different masses and forces on different gravity settings.

self.physicsBody?.mass = 2.0
let force = CGVector(dx: 0.0, dy: -5.0 * 150.0 * 2)

---

self.physicsWorld.gravity = CGVector(dx: 0, dy: 15.0)
self.physicsBody?.mass = 2.0
let force = CGVector(dx: 0.0, dy: -15.0 * 150.0 * 2)

and they all worked find. F=ma.

Question is the mysterious factor of 150. Where the hell does it come form?

Answer 1

OK, so it was all about wrong documentation of Apple. Here's the truth of 150.

this seems to be little bit stupid, but applyForce is measured in ((points * kilo) / sec ^ 2), but the gravity acceleration is in Newtons ((kilo * meter)/ sec ^ 2) (despite the fact it's described as meters per second in documentation. Meters per second! Acceleration!). Multiply it by mass and get the force.

https://stackoverflow.com/a/31868380/5752908

Come on, Apple... It's been out there for 4 years.