Catapult Arm rotation - libgdx and box2d

Question

I'm currently working on a catapult game with box2d but I have encountered a problem regarding the rotation of the arm of the catapult.

Let me explain a little bit about my catapult:

I have 2 objects. * The catapult arm - Dynamic Body * The catapult base - Static Body

I want the catapult arm to rotate back and forth within a certain range. Here is an image of my catapult: http://i.gyazo.com/7ce4dfa87b2083a0eb62df7c4acd3e47.png

Now, I tried to "link them" using joints but it didn't work. Initially, I want the arm to be bent a little bit to the left. I have my own Game World object that contains the Box2d world object and all the game objects. Moreover, I have an object that is called Game Renderer which takes care of the rendering in the game. Here is what I have tried so far in my Game World object:

package com.david.gameworld;

import com.badlogic.gdx.math.Vector2;
import com.badlogic.gdx.physics.box2d.World;
import com.badlogic.gdx.physics.box2d.joints.RevoluteJointDef;
import com.david.objects.CatapultArm;
import com.david.objects.CatapultBase;

public class GameWorld{

    private World b2dWorld;
    private CatapultArm arm;
    private CatapultBase base;
    private RevoluteJointDef jointDef;
    public GameWorld() {
        // Initialize The Box2d World
        b2dWorld = new World(new Vector2(0,0),true);
        initializeObjects();
    }
    public CatapultArm getArm() {
        return arm;
    }
    public CatapultBase getBase() {
        return base;
    }
    public World getBox2DWorld() {
        return b2dWorld;
    }
    public void updateWorld(float delta) {
        b2dWorld.step(1/delta, 8, 5);
    }
    private void initializeObjects() {
        // Initialize The Objects(Catapult, Base)
        arm = new CatapultArm(b2dWorld);
        base = new CatapultBase(b2dWorld);
        jointDef = new RevoluteJointDef();
        jointDef.initialize(arm.getBody(), base.getBody(), new Vector2(120,62));
        jointDef.enableMotor = true;
        jointDef.enableLimit = true;
        jointDef.motorSpeed = -1260;
        jointDef.lowerAngle = (float) Math.toRadians(90);
        jointDef.upperAngle = (float) Math.toRadians(75);;
        jointDef.maxMotorTorque = 4800;
        b2dWorld.createJoint(jointDef);
    }
}

CatapultArm and CatapultBase are both objects in my game that contains all the configurations of the bodies(position,type,etc).

Here are the codes:

CatapultBase.java

package com.david.objects;

import com.badlogic.gdx.physics.box2d.BodyDef;
import com.badlogic.gdx.physics.box2d.World;
import com.badlogic.gdx.physics.box2d.BodyDef.BodyType;

public class CatapultBase extends ObjectInWorld{

    public CatapultBase(World world) {
        super(world);
        initiate();
    }
    @Override
    protected void initiate() {
        //The Body
        bodyDef = new BodyDef();
        bodyDef.type = BodyType.StaticBody;
        bodyDef.position.set(140f, 62f);
        body = world.createBody(bodyDef);
    }
}

CatapultArm.java

package com.david.objects;

import com.badlogic.gdx.physics.box2d.BodyDef;
import com.badlogic.gdx.physics.box2d.BodyDef.BodyType;
import com.badlogic.gdx.physics.box2d.World;

public class CatapultArm extends ObjectInWorld{

    public CatapultArm(World world) {
        super(world);
        initiate();
    }

    @Override
    protected void initiate() {
        //The Body
        bodyDef = new BodyDef();
        bodyDef.type = BodyType.DynamicBody;
        bodyDef.position.set(154f, 62f);
        body = world.createBody(bodyDef);
    }
}

ObjectInWorld.java

package com.david.objects;

import com.badlogic.gdx.physics.box2d.Body;
import com.badlogic.gdx.physics.box2d.BodyDef;
import com.badlogic.gdx.physics.box2d.World;

public abstract class ObjectInWorld {

    protected BodyDef bodyDef;
    protected Body body;
    protected World world;
    protected ObjectInWorld(World world) {
        this.world = world;
    }

    protected void initiate() {

    }
    public float getX() {
        return body.getPosition().x;
    }
    public float getY() {
        return body.getPosition().y;
    }
    public Body getBody() {
        return body;
    }
}

How can I "pin" the catapult arm to the base so that it would rotate nicely?

it suppose to bent slightly to the left because of the limits I set up(9 degrees to 75 degrees) but it doesn't. I think I have a problem with the way I use coordinates. — David Lasry
Well what does it do instead? You have the lower limit set higher than the upper limit, that is generally not a good idea. — iforce2d

iforce2d iforce2d · Accepted Answer · 2014-09-28T05:30:34

Your bodies don't have any fixtures on them. In the original C++ version at least, this would cause them to have no density, and therefore no mass, so they would be treated as fixed rotation.

Catapult Arm rotation - libgdx and box2d

1 Answers