A ball will sit patiently upon the half-circle at the end of a track. If you are making your own track from cardboard, use scissors, or better yet, a large hole-puncher if you have one, to make the half-circle.
The forces of gravity and friction keep the ball in place. That’s Newton’s first law – the law of inertia. That means that a body (an object) at rest tends to stay at rest, and a body in motion tends to stay in motion – with the same speed and in the same direction, until some other force makes it change. Like a kid who doesn’t want to get out of bed in the morning (body at rest), and then insists on staying up late at night (body in motion). Whatever force the parent exerts needs to be greater than the force keeping that kid asleep, or awake!
We have stored potential energy. The patient ping-pong ball waits patiently till some force or object transforms its potential energy into kinetic (movement) energy. And so, I am reminded to be patient with myself, especially when I am failing, which happens a lot, and in failing again and again, I am storing more and more of my own potential energy for me to release in my quest to understand how the world works, and to help make it work even better.
All complicated machines are made from simpler parts. If you haven’t mastered this technique, or don’t yet understand it, break it down into smaller parts till you “get” each one, and then build up from there. Find or make a track with a half-circle at the end. Lean the track to make a ramp. Place a ball gently upon the half-circle. Change the size of the half-circle if needed. Let one ball tap the next. Repeat.