Box2D: Rope pendulum (no stop)

Question

I would like to make a scene where there is a "pendulum" that oscillates continuously, no stop. I have uploaded an image for better clarity. So i try to use Box2D joints. For example:

   RevoluteJointDef revDef = new RevoluteJointDef();
     revDef.initialize(ball, box, ball.getWorldCenter());
     revDef.lowerAngle = 0 * MathUtils.degreesToRadians;
     revDef.upperAngle = 180 * MathUtils.degreesToRadians;
     revDef.enableLimit = true;
     revDef.maxMotorTorque = 10.0f;
     revDef.motorSpeed = 2.0f;
     revDef.enableMotor = true;

  revoluteJoint = (RevoluteJoint)world.createJoint(revDef);

But it doesn't work. If i comment limits and motor lines i obtain the same result that i obtain when these lines are uncommented. Although motor is enabled, it seems not work.

P.S. The motor must stop when the user releases the box by press a button. So the box falls to the ground due to the force of gravity.

Can someone help me? Thanks!!

Scene image

See also this [example](http://stackoverflow.com/a/11233735/230513) using `FloatSpring`. — trashgod, Jan 21 '14 at 18:36

score 1 · Accepted Answer · edited May 23 '17 at 12:30

I don't think you need a revolute joint for this, but a rope joint (b2RopeJoint). A revolute joint will make the two object rotate around a single point. The rope joint will keep one swinging from another like a pendulum.

You need to make the pendulum attach with a single rope joint to a static body. Then cut the rope joint when you want it to fall. If gravity is turned on and you don't have any retarding forces, the pendulum should continue indefinitely (or for a really long time depending on numerics).

Take a look at this post that was just done for something like this. Note that the code is also posted on github here. In that case, there were two extra rope joints added to constrain the body so that it could not move beyond the end of the initial swing. I don't think you need those.

To create the pendulum yourself, use something like:

   // Calculate the local position of the
   // top of screen in the local space
   // of the ground box.
   CCSize scrSize = CCDirector::sharedDirector()->getWinSize();
   b2Vec2 groundWorldPos = b2Vec2((scrSize.width/2)/PTM_RATIO,(scrSize.height)/PTM_RATIO);
   b2Vec2 groundLocalPos = m_pGround->GetLocalPoint(groundWorldPos);

   // Now create the main swinging joint.
   b2RopeJointDef jointDef;
   jointDef.bodyA = m_pGround;
   jointDef.bodyB = body;
   jointDef.localAnchorA = groundLocalPos;
   jointDef.localAnchorB = b2Vec2(0.0f,0.0f);
   jointDef.maxLength = (groundWorldPos-body->GetWorldCenter()).Length();
   jointDef.collideConnected = true;
   world->CreateJoint(&jointDef);

NOTE This is in C++, not java (for libgdx), but the approach should be sound and you just have to map the "->" onto the "." where needed.

In my example, it ended up looking like this (image liberated from other posted answer):

enter image description here

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Thanks @FuzzyBunnySlippers! It's the 2nd day that i work on jBox2D... I should "take confidence" yet. — fpellegrino, Jan 18 '14 at 17:49
@fpellegrino Best of luck to you. Feel free to look at some of the other posts on my site(http://www.nlideas.com). Also, there are a lot of good tutorials on the iForce2d site (https://www.iforce2d.net/b2dtut/). — FuzzyBunnySlippers, Jan 18 '14 at 19:00
How can I draw the rope joints in my gameplay? Obviously, if I remove the DebugRender the rope and the other body models are hidden. So for the some element of the game I use images (sprite) but for the rope? The sprite should be the same position and obliquity of the joints... — fpellegrino, Jan 22 '14 at 22:23
To do this, you can use the "verlet" approach, which allows for slacking lines (e.g. "cut the rope" game) or you can do the "quick and dirty" way: Create a "rope sprite" image (small circle with some coloring), then replicate it at even intervals along the length of the rope, usually so they overlap. You have to recalculate the positions of the sprites if the rope is moving (each frame). I have an example of this here (see video on main page):http://nonlinearideas.com/. — FuzzyBunnySlippers, Jan 22 '14 at 23:36

Box2D: Rope pendulum (no stop)

1 Answers1