ApplyForce or ApplyImpulse not working the same on device

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ApplyForce or ApplyImpulse not working the same on device

(6 posts) (3 voices)

Started 2 years ago by adpro
Latest reply from arutema

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adpro
Member

Hello,

I'm using Box2D and ApplyForce - I have tried this with ApplyImpulse as well.

Basically what is happening is this....

I have a force being added to an object. I grab a bunch of info from the user (vertical and horizontal strength are the two I use for the force).

I use this information to apply a force, and I'm doing it like this...

b->ApplyForce(b2Vec2((20*horStrength)*b->GetMass(), (20*vertStrength)*b->GetMass()), b->GetPosition());

This works fine on the simulator and does what I would expect. A weak force is applied on the phone, but it doesn't seem to increase as I increase the value of horStrength or vertStrength.

Anyone know what could be going on?

It looks like the same is happening to the rotation as well. Works fine on the simulator, but seems to always apply the same rotation no matter what I set my variables to.

Posted 2 years ago #
adpro
Member

A little more on this...

I tested some stuff out and figured out that there must be some kind of a limit on how much force can be applied.

I have a ball that I want to fly across the screen in a similar gravitational envoronment to earth. Lets say a baseball.

Now, I want the ball to be able to be wipped across the screen pretty quickly.

I have tried this with SetLinearVelocity as well, but it seems to cap at about the same as ApplyForce.

In other words, if I use small numbers for the force or the velocity, it will not go very far before hitting the ground. If I increase this to about 20.0, increasing it any more doesn't seem to make much difference.

So, for the following

b->ApplyForce(b2Vec2(20, 20), b->GetWorldCenter());

I get the same result as

b->ApplyForce(b2Vec2(2000, 2000), b->GetWorldCenter());

I tried changing my world gravity to be lower, but this makes things look so slow.

So, is there a way to speed things up in Box2D, but use the same gravity (if I lower the gravity, I can get it to work the way I want, but it's just really slow) or is there something else I am missing here? Is there a cap to these force functions?

Thank you very much for any input.

Posted 2 years ago #
MrMojo
Member

I've got no idea what's up, but if you don't get an answer here, you could try the Box2d forums as well, as it seems to be mostly a box2d issue, I'd say. good luck!

http://www.box2d.org/forum/

Posted 2 years ago #
adpro
Member

yeah, that's kind of what I was thinking after I tested it a bit more.

Thanks for the reply!

Posted 2 years ago #
adpro
Member

I thought I would kind of explain how I fixed these issues.

It ended up being related to a change in Box2D where a newer version doesn't let you go faster than about 500km/h or something. I didn't test in the simulator after I upgraded cocos2d (I didn't realize I was still using an old version of 8.1 or something until recently, so I upgraded). Either way, the simulator was doing the same thing. So, ignore the title of my thread, hehe.

In Box2D, as many of you know, the units of measurements are in meters. The iphone uses pixels.

When I was doing my conversions, I was dividing everything by 32. Basically making it so 32 pixels equals about 1 meter. I had a ball that was about 20 pixels in circumference. So, it was 2/3 of a meter.

When I would throw it at speeds that should make it look normal relative to everything else, it didn't go very far. This is because of the conversion.

So, I changed my number to 100. 100 pixels = 1 meter now. So, the ball is about 20cm in circumference. Now it does what I would expect.

A good tutorial to check out is here.... I think lesson 5 explains this better....

http://kerp.net/box2d/index.php

Posted 2 years ago #
arutema
Member

use this
b->ApplyImpulse(b2Vec2(2000, 2000), b->GetWorldCenter());

apply force act on some static varaibles

Posted 2 years ago #

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