A cannon is mounted on the back of an ice sled. Together, the cannon and sled have a mass of 4.0 x 10^3 kg. If a 50.0 kg shell is now loaded into the cannon and fired horizontally at 150 m/s, what is the speed of the sled after firing the cannon?

Independent variable is something that is changed in a experiment. 

I'm having some difficulty seeing the diagram clearly.but the question says there's a net inward force on the object.so we know right away,  from newton's 2nd law of motion, thatthat baby is accelerating.  it doesn't even matter which way thenet force is pointing.  if there's any net force, then either theobject's speed or else its direction must be changing.

The actual answer is c force stays the same while direction changes