Add a helmet.
Optional to overlay a position-grid on the real space (to correlate with graph).
    Like moving man, force1d or ramps, disabled by default.
Put coefficient of friction in an advanced button.
Zoomable Energy vs Position graphs.
Bug: Setting record, then live, then record resets to last record state.
Add clipping to energy vs position plot (so data doesn't fall outside the data area).
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

Maybe "Live" should be changed to "Go" instead of "Play"
Bug: clear button erroneously shows 'this simulation is paused' when it is not.
 Bug: The measuring tape's initial readout is incorrect.

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