How Can I Trigger a Ball While the Car is Moving for My Physics Experiment?

[nese]

For my physics project,i decided to do an experiment that shows horizontal velocity remains constant.There is a video about it
it seemed easy at the beginning but now I am in trouble with how to triggle the ball while the car is moving.I hope u can give me some advice and help me with my project.

 

[Bystander]

What is the exact difficulty you're having?

 

[haruspex]

how to trigger the ball while the car is moving.

I see no way to advise on that without knowing the details of your equipment. How is it triggered when the car is stationary? Do you have the same equipment as in the video?