Answer:
The length of the string is 0.051 meters
Explanation:
It is given that,
Tension in the string, T = 240 N
Mass of the string, m = 0.086 kg
Speed of the wave, v = 12 m/s
The speed of the wave on the string is given by :

M is the mass per unit length of the string i.e. M = m/l.......(1)
So, 

M = 1.67 kg/m
The length of the string can be calculated using equation (1) :


l = 0.051 m
So, the length of the string is 0.051 meters. Hence, this is the required solution.
Answer:
v = 10 [m/s].
Explanation:
The largest mass is that of 4 [kg], in this way the momentum can be calculated by means of the product of the mass by velocity.

where:
P = momentum [kg*m/s]
m = mass = 4 [kg]
v = velocity = 5 [m/s]
Now the momentum:
![P=4*5\\P=20[kg*m/s]](https://tex.z-dn.net/?f=P%3D4%2A5%5C%5CP%3D20%5Bkg%2Am%2Fs%5D)
This same momentum is equal for the other mass, in this way we can find the velocity.
![P=m*v\\20=2*v\\v=10[m/s]](https://tex.z-dn.net/?f=P%3Dm%2Av%5C%5C20%3D2%2Av%5C%5Cv%3D10%5Bm%2Fs%5D)