1. Rolling Question :
A sphere S rolls without slipping mving with a constant speed on a plank P.The friction between the upper surface of P and the sphere is sufficient to prevent slipping , while the lower surface of P is smooth and rests on the ground.Initially ,P is fixed to the ground by a pin N.If N is suddenly removed :

[A] S will begin to slip on P
[B] P will begin to move backwards
[C] The speed of S will decrease and its angular velocity will increase
[D] There will be no change in the motion of S and P will still be at rest.
Answer given : [D] !
IMPORTANT POINT / DOUBT : I just cant understand where the reaction (of the friction force on the sphere) acting on P vanishes , why is it not able to make P move ??? [7]
2.Extremely BASIC question from Waves [I'm poor at Mechanics :( ]
A string of length L is stretched by L /10 and the velocity of transverse wave along it is v .When it is stretched by L/20 [assuming Hooke's Law is applicable ] , the velocity of wave will become :
[A] v
[B] √2 v
[C] v√2
[D] 2v
Answer given : [C ]
IMPORTANT POINT / DOUBT : This answer will come if we use the relation v is directly proportional to √T [ T: Tension , v: velocity] and then proceeding by Hooke's law..BUT how can we say that v is directly proportional to √T when length of the medium changes ?? [7]
3.ANOTHER DOWN TO EARTH BASIC QUESTION from Laws of Motion :
In the figure given below ,the spring ,the pulleys and the strings are all massless.The pulleys are frictionless at their axles and it is given that m1> 2m2.The system is initially in equilibrium and the string connecting m2 to the rigid support below is now cut.The initial acceleration of m2 immediately after cutting the string will be :
Figure :

[A] (m1-2m2)2m2 g
[B] (m1-m2)m2 g
[C] (2m1-m2)m2 g
[D] (m2-m1)m2 g
Answer given : [A]
MY DOUBT: HOW ?..PLEASE GIVE FULL STEPS..PLEALSE !