This is called the Phi Phenomenon.
This is an illusion of movement created when two or more adjacent lights blink on and off in quick succession; when two adjacent stationary lights blink on and off in quick succession; we perceive a single light moving back and forth between them. It is an optical illusion of perceiving a series of still images, when viewed in rapid succession, as continuous motion.
Answer
given,
Tension of string is F
velocity is increased and the radius is not changed.
the string makes two complete revolutions every second
consider the centrifugal force acting on the stone
= 
now centrifugal force is balanced by tension
T =
From the above expression we can clearly see that tension is directly proportional to velocity and inversely proportional to radius.
When radius is not changing velocity is increasing means tension will also increase in the string.
Answer: It is not likely.
Explanation:
When the bus is moving forward, all the objects inside of it also are moving forward.
Now, as the objects inside the buss are not fixed to the bus, if the bus suddenly stops the objects inside of it will keep moving forward, because of the conservation of the momentum, defined as the quantity of motion (Similar to when you are in a car and it suddenly stops, you can feel the forward impulse).
Then is not likely that, in a case where the bus stops suddenly, an object inside the bus flies backward in opposite direction to the previous movement of the bus.
