Answer:
15.10m/s
Explanation:
The mass of the ball(m)=400g = 0.4kg
The radius of the string is(r)=15m
The tension in the string is(T)=10N
The acceleration due to gravity = 
The tension in the string when the body is at the bottom is given by

To find the speed of the ball, we make v the subject of the formula
Therefore, 



The speed of the ball = 15.10m/s
by energy conservation we know that
KE or rotation + KE of translation = gravitational PE
now we have

also we know that

now we have



now when it is rolling on ground the torque acting on it due to friction force is given by




now angular speed at any time is given as


so above is the angular speed in terms of time "t"
The electron fell down to the lowest orbital it could inhabit
The wavelength would decrease because frequency is inversly proportional to the wavelength provided the speed is constant: f=v/λ