Pulling a sled with an angled force (with friction)

Description : A sled is pulled along a horizontal surface by a force that can be angled up from the horizontal. There is friction between the sled and the ground, and the two coefficients can be changed. Change the values below, then click the UPDATE CONDITIONS button - then PLAY.

  Force magnitude =   N   Angle up from +x axis =   degrees
  mass of sled =   kg
  coefficient of static friction µs =       coefficient of kinetic friction µk =   (less than µs!)      

Some possible investigations: (There are many possible solutions to these, depending on the values.)
A) For a given kinetic coefficient of friction, and a fixed pulling force magnitude (assuming it is enough to pull the block free and have it accelerate) - what happens to the ax as you change the angle?
B) In the above conditions, can you find the best angle, to get the maximum acceleration sideways (can you calculate that angle also)? (Conceptually, why should there be a best angle? - think of what happens to all the various forces as the angle increases .. as it decreases.)

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