Box2D Forums

Hi all,

I have a platform and an object at rest on top of it. The platform is a static object. They both go to sleep as expected. When I move the platform horizontally (by setting it's position directly) the platform wakes. However the object which is resting upon it doesn't. The platform can slide out of the way without any effect on the object and as it is still asleep, when the platform has moved out of the way the object doesn't fall.

A second instance of this occurs with a sliding platform and a ball on a slight slope. The ball rolls into the platform and comes to rest. They both sleep. The platform wakes and slides to push the ball, but instead passes through the ball. If the ball comes to rest without touching the platform the platform then wakes the ball. I.e. it is only objects that are touching that don't wake each other, I'm guessing because there is no new contact created when the static object moves.

I know I could manually wake the other object but I would prefer if this happened automatically. Is there any settings or specific methods I'm missing for this to happen?

In general, moving a body by modifying the positions directly tends to produce some undesired results in the physics. While working on my game, I had encountered problems with sleeping bodies and them not waking up, even when you change their position directly.

It seems like you're moving the static bodies by changing their positions directly to produce a sliding effect while keeping the platforms completely sturdy, uninfluenced by any force. If you want a moving platforms that are not influenced by any other body, use the kinematic setting (b2Body.b2_kinematicBody) instead of static when you're defining the body definition and use SetLinearVelocity() to move the platforms. The kinematic bodies do not collide with static or other kinematic bodies and they are not influenced by gravity forces or any other forces. They do not change velocity until you use SetLinearVelocity() again.