Add a movable PE=0 level (graphic & model).
Wash out the images, they are still distracting.
Put coeff/friction in an advanced button.
Zoomable Energy vs Position graphs.
Record bug:
	Record, then live, then record resets to last record state.	
Add clipping to energy vs position plot.
Add game-ish modes to sk8r application
	. 2-skater mode
	. Grab the apple?
	. Jump the buses?
Bug: When user drags track that skater is resting on, the skater doesn't move with it.
[internal] Figure out the meaning of the magic grab number in EnergyConservationModel
Add a helmet.
Optional to overlay a position-grid on the real space (to correlate with graph).
Maybe "Live" should be changed to "Go" instead of "Play"

low priority:
User-selectable skater/vehicle graphics.
Crashing skater?