Physics involved in helping little kids go sledding

Last week I babysat for 3 little kids. Their Dad build them a sledding hill out a slide. I took them outside after bundling them up and we went sledding. When sledding I noticed how easily the sled glided down the snow bank, then I thought, snow and ice have a low coefficient of friction. Additionally, I noticed that when the kids went down individually, they went quicker down the hill. Because Force equals mass times acceleration, acceleration is also equal to force divided by mass. Therefore, the kids went faster when they were by themselves because the mass was less and the acceleration could grow. Lastly, The kids dad also put a ramp at the end of the hill which slowed down the sled and changed its velocity. The change in velocity was helpful because without stopping the sled could of gone in the large tree in the backyard, because an object in motion tends to stay in motion unless acted upon by an outside force.