Calculate speed from velocity vector using box2d GetLinearVelocity();

Question

I need to find the speed of an object in a game. The game is made in HTML5 with jquery and jquery.box2d. For this I can use these methods:

 GetLinearVelocity().x;
 GetLinearVelocity().y;

I'm then trying to calculate the speed from this piece of code, but get some values that doesn't make sense when I console.log it. This is my code:

 var heroVelX = game.currentHero.GetLinearVelocity().x;
 var heroVelY = game.currentHero.GetLinearVelocity().y;

 var speed = Math.sqrt(heroVelX^2 + heroVelY^2);
 console.log(speed);

Some of the values in console.log are numbers, but most of them are NaN (Not-A-Number), which confuses me? Can someone help me solve this?

The goal I want to achieve, is to see when the speed(of object .currenHero) drop below a certain value, so I can excute a new state in the game.

score 1 · Accepted Answer · edited May 23 '17 at 12:27

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Your problem is that you're using the wrong operator (Bitwise XOR) for doing square - see here.

What you need to do is:

var speed = Math.sqrt(Math.pow(heroVelX, 2) + Math.pow(heroVelY, 2));

edited May 23 '17 at 12:27

Community

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answered Oct 19 '14 at 10:36

ydaniv

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right answer, but `Math.pow(foo, 2)` is overkill in the circumstances compared to just writing `foo * foo`. – Alnitak Oct 19 '14 at 10:39
@Alnitak whatever your style may be, this is the most straight forward way in JS. – ydaniv Oct 19 '14 at 11:24
I see your point with `foo * foo` being more simple, just trying to get a positive value. – Tommy Otzen Oct 19 '14 at 11:25
@ydaniv no, the most straight-forward way is merely `var speed = Math.sqrt(heroVelX * heroVelX + heroVelY * heroVelY)` – Alnitak Oct 19 '14 at 12:04
You can just do `heroVelX ** 2` now anyway. – Apr 28 '22 at 22:35

George · Answer 2 · 2014-10-15T20:36:02.347

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The only time the square root function should return NaN is when the value being square rooted is negative. A way to go about testing if this is the issue would be to try squaring the values in a different line of code before square rooting them.

heroVelX = (heroVelX) * (heroVelX)

Another way to potentially shine some light on the problem would be to add log statements printing out the values of the velocities and the velocities squared before square rooting.

edited Oct 15 '14 at 20:36

answered Oct 15 '14 at 20:23

George

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I do get negative values if I log the the input before I do Math.sqrt. Do jqeury have a way to convert tve values to only positives? – Tommy Otzen Oct 15 '14 at 21:51
Taking the absolute value will return the positive number with the same magnitude. So Math.abs(-10) = 10 – George Oct 15 '14 at 21:58
Thanks George. I'll try that. – Tommy Otzen Oct 15 '14 at 22:00
-1 for missing the utterly bleeding obvious misuse of the `^` operator. – Alnitak Oct 19 '14 at 10:39

Calculate speed from velocity vector using box2d GetLinearVelocity();

2 Answers2