First time project, motor with retractable gearing?

I just got a 3D printer and have decided to try something mechanical in design. I want make a Mario themed hot wheels track for my nephew and I want make a section of Thwomps which are big rocks that try to smash your car. I want the Thwomps to basically be hooked to a geared shaft that connects to a geared motor. I want the gear to either completely disengage from the shaft or disengage internally like a clutch and free spin so that the shaft and Thwomp can fall down along some guides and then the gear re-engage the shaft and retract the assembly back up. The problem is I am not sure what I need to mechanically accomplish that. Any suggestions or guidance would be super appreciated.

Hello.

Some of our motors and mechanical components might be useful for something like that, but we do not have any specific suggestions for how to build something like that. My best suggestion is that you search for an example of something similar to what you are trying to design, and then you can more confidently determine what you need from there.

- Patrick