Rolling down the hill

After our last visit to Merri Creek, several children were curious about how their body position affected the way that they rolled down the hill next to the velodrome.  

"I was wondering why when you roll down the hill it's really fast.  I know it's steep when you're running, but I don't know why it's fast when you're rolling."

"I was wondering why when you roll a little bit sideways and a little bit down you go slower, and when you go straight down you go faster"

"I was wondering why when I went straight down on the hill, I didn't move, when I was pointing down the bottom of the hill I was wondering why I didn't move."

"Well you can't roll down if there's a flat surface like this, so I think he was on the flat so he wasn't rolling down." 

When we got back to school, we decided to explore these questions in a bit more depth, so we re-created the velodrome hill using some old bookshelves and some mini-me's out of cardboard rolls. 

We found that even when the mini-me was pointing straight down, just like at the velodrome, the roll slid straight down the hill.  Several students suggested that the problem was that it wasn't enough "grippy" enough, unlike when we were rolling on the grass:

"Maybe it's because there's grass surrounding all around on the hill and there's no grass on here."

So we added some blackboards and blu tack to our model to make it more realistic.  

Once the model was modified to work realistically, the children tested the various positions that made them roll more or less effectively.  Then re-created each scenario and hypothesized why certain positions slowed them down so dramatically.  They identified various forces including weight, gravity and shape that might have affected the way they rolled: 

"I think when he put it like that there was more weight."

"It's because the gravity is pulling down, maybe there's not enough gravity when I'm head first, maybe when I'm going sideways there's more gravity pulling me down."

This lead to a discussion of surface area - "how much of me is facing down" - and whether or not this affected by gravity.  We decided that it wasn't the issue, because the Earth has the same gravity all the time, and that it probably wasn't weight, because we didn't seem to get any bigger or heavier when we changed positions.

Gravity, and how it can be cheated, is obviously a fascinating subject for the children.

"Gravity pulls us down, it pulls us down when we jump."

"Why do birds fly with gravity when we can't?"

"Why can planes fly?"

Eventually we decided that the rolling worked best sideways "because it's not round when you're like this but now it's round."  

We then experimented with a couple of different shaped mini me's to check how well different shapes roll, and several children continued with the experiment in project time.