I don't know for sure but i think that it is because the force him pushing her pushed him back as well
Hope i helped
Answer:
3.46 seconds
Explanation:
Since the ball is moving in circular motion thus centripetal force will be acting there along the rope.
The equation for the centripetal force is as follows -
Where,
is the mass of the ball,
is the speed and
is the radius of the circular path which will be equal to the length of the rope.
This centripetal force will be equal to the tension in the string and thus we can write,

and, 
Thus,
m/s.
Now, the total length of circular path = circumference of the circle
Thus, total path length = 2πr = 2 × 3.14 × 2 = 12.56 m
Time taken to complete one revolution =
=
= 3.46 seconds.
Thus, the mass will complete one revolution in 3.46 seconds.
Force exerted by the bullet = mass * acceleration = 0.013 * 850 = 11.05 Newtons.
the rifle exerts same force in opposite direction so we have
11.05 = 3.5 * a
acceleration = 11.05 / 3.5 = 3.16 m /s^-2
3.85 pounds is the answer