A toy airplane, attached to a force meter by a string, flies in a horizontal circle. Design two independent experiments to determine the magnitude of the sum of
forces (the net force) exerted on the airplane when it flies in a circle.

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.