Why is force dependent on acceleration, not velocity? According to $F = ma$, force is a result of acceleration and mass, right? 
However, I don't understand why velocity is not used instead of acceleration. A train moving at 100 miles/hour will still impart a great force on you even though it has no acceleration. Further, dropping a book at 10ft will impart a greater force on the gorund than dropping it at 1ft. So it seems that velocity would influence the force more than the acceleration would.
Why is this not the case?
 A: The $F$ in the equation $F=ma$ is not the force that would be exerted by the object if it were to hit something else. Instead, $F$ represents the net force acting on the object that must be present in order to produce the current acceleration $a$ of the object. A better way to write Newton's second law is 
$$F_\text{net}=ma,$$
since it shows explicitly which force is being represented on LHS of the equation is.
In your train example, if the train is traveling at a constant velocity of 100 mph, the acceleration is zero, and by Newton's second law the net force is also zero. But this has no bearing on what force the train would exert on something if it collided.
A: This is so simple.....
Two trains traveling at 100 mph= no acceleration- agreed.
The resulting force of said train would depend on the 
Mass/density of the opposing object- newtons second law.
Since we don't know the speed of the opposing object, it can only be presumed that the opposing force is equal.....mork calling orson, come in orson
A: HERE FORCE STANDS FOR THE FORCE ACTING ON THE TRAIN TO PRODUCE AN ACCELERATION.SINCE IT HAS NO ACCELERATION,IT DOES NOT REQUIRE ANY FORCE TO KEEP IT GOING(WITHOUT CONSIDERING THE RESISTANCE OFFERED BY AIR).
                                                  IT DOES NOT MEAN THAT IT WILL PRODUCE A FORCE.IT HAS NOTHING TO DO WITH COLLISIONS.FOR COLLISIONS,YOU HAVE TO CONSIDER MOMENTUM(MV)....HOPE THIS IS USEFUL
