Physics lesson #2:
"Moving objects accelerating at "x speed" on the "y axis" that meet a non-moving mass will ultimately cease locomotion at a varying rate, although more often than not, abruptly. Depending on mass of said object, and force (force is a push or pull on an object resulting on the object's interaction with another object "contact force"), the resulting impact "can/should/will" stimulate the cerebral cortex of the brain and process this stimulus for future reference.
Continued repetition of "contact force" will lead to total loss of cerebral stimulus and thus leave the mountain bike-E with an incurable state of mind known as "Doug Long Syndrome!"
The cure is: ride long, ride fast, pick yourself up and keep riding!