Some more questions

Okay, I have an object that I want to stay in a certain place in the screen, and not be subject to gravity or forces from other objects (like as if it has a mass of 0), but still things can bounce off it.

The trouble is I can’t use a mass of 0, cause I want to be able to move it at certain times, i.e. have it slide back and forth

How would I go about doing this?

Also, I want to create a wheel, but not on a vehicle, I want the wheel to be jointed to the world, and be only able to turn about its centre and start spinning when things catch it on its edge. What sort of join should I use and how would I go about setting up the constraints to get this behaviour?

Material callbacks should be used for both things. The joint for the second thing should just be a simple hinge.

can you ellaborate ?

Sorry I was so short last time, rather busy.

Okay, I have an object that I want to stay in a certain place in the screen, and not be subject to gravity or forces from other objects (like as if it has a mass of 0), but still things can bounce off it.
Change its forceCallback from the default to your own that does not add gravity.
Make a Material for it that causes forces to still work on things that 'bounce' off of it, but that nullifies all forces to it itself.

Also, I want to create a wheel, but not on a vehicle, I want the wheel to be jointed to the world, and be only able to turn about its centre and start spinning when things catch it on its edge. What sort of join should I use and how would I go about setting up the constraints to get this behaviour?
I have not actually made wheels yet (most things I make hover), but I'd wager you'd just make a cylinder/sphere/whatever for the wheel, and attach a hinge joint (or whatever name such a one would use) to its center, binding it to the world point (wheel will never translate, but it will rotate around the one axis the hinge join allows).
You would, again, make a Material callback that acts based on if something touches the wheel, you could add forces to make it start rotating for example.