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# Dynamic gravity vector

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### #1jbosch  Members   -  Reputation: 107

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Posted 21 May 2011 - 09:34 AM

Hi,

I have a physics engine in my game. I have to set him the gravity vector to initialize it. That would normally be x,y(0,10). Ok, easy one. But what happens if the container is a mobile, and I want to modify the gravity vector according to the accelerometer information? This is, if I give the mobile phone a 90º inclination, the "ground" is now different than in its initiajl state, I want to reflect this, but at any angle, not just 90º movements.

This can be difficult to explain, so I have attached a picture

Any idea?

### #2Hodgman  Moderators   -  Reputation: 49099

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Posted 21 May 2011 - 09:38 AM

Does your physics engine provide a function for setting a new gravity vector? Is the question how to change the gravity vector, or how to figure out what the new vector is?
Take your default gravity vector and rotate it by the desired angle.

### #3jbosch  Members   -  Reputation: 107

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Posted 21 May 2011 - 10:28 AM

thanks Hodgman,

The question is more about how to calculate this angle.

The accelerometer gives me some information. For example, when the phone is landscape, the acceleration (x,y) its (1,0), and when I put the phone in portrait position, its (0,1).

Any idea?

Thanks!

### #4japro  Members   -  Reputation: 887

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Posted 21 May 2011 - 11:16 AM

So the accelerometer always gives you a normalized vector to the "down"-direction? Then just multiply that vector with the magnitude of your g.

### #5jbosch  Members   -  Reputation: 107

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Posted 21 May 2011 - 11:36 AM

So the accelerometer always gives you a normalized vector to the "down"-direction? Then just multiply that vector with the magnitude of your g.

whaaw, you are right. Very simple! I feel dumb now jajaja

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