In a primary school class demo' rolling a car down a ramp, between lightgates, adding masses each time. the time between two points DECREASED with inceasing mass. The car had friction drive rear wheels, so had some inerita to overcome.

Can anyone please help with a good explanantion for the 10-11 yr old children. I believe that with a  free wheeling car added mass should not make any difference?
Thanks,
Neil
UK