The device you are referring to is called a "release mechanism" or a "ball launcher." It is commonly used in physics experiments or demonstrations to release a ball from a stationary position on an inclined plane.
The device commonly used to hold and drop a ball down an incline plane from a stationary position is called a "ball release mechanism." This mechanism typically consists of a rod, lever, or trigger mechanism that holds the ball in place at the top of the incline. When triggered, it releases the ball so that it rolls down the slope.
