Given:
F_gravity = 10 N
F_tension = 25 N
Let's find the net centripetal force exterted on the ball.
Apply the formula:

From the given figure, the force acting towards the circular path will be positive, while the force which points directly away from the center is negative.
Hence, the tensional force is positive while the gravitational force is negative.
Thus, we have:

Therefore, the net centripetal force exterted on the ball is 15 N.
ANSWER:
15 N
<u>Answer :</u>
(a) d = 0.25 m
(b) d = 0.5 m
<u>Explanation :</u>
It is given that,
Frequency of sound waves, f = 686 Hz
Speed of sound wave at
is, v = 343 m/s
(1) Perfectly destructive interference occurs when the path difference is half integral multiple of wavelength i.e.
........(1)
Velocity of sound wave is given by :




Hence, when the speakers are in phase the smallest distance between the speakers for which the interference of the sound waves is perfectly destructive is 0.25 m.
(2) For constructive interference, the path difference is integral multiple of wavelengths i.e.
( n = integers )
Let n = 1
So, 


Hence, the smallest distance between the speakers for which the interference of the sound waves is maximum constructive is 0.5 m.
1.6764 meters. You can also google inches to meters for your answer.
We don't know that at all. The 3rd law says that the REaction is opposite and EQUAL to the action. We don't know where that "twice as much" comes from.