Question

physics a problem from nLM help me plzz

Answer 1

Option  C is correct.

I get : The speed of the pulley P1 is downwards and is 5 m/s.
          The speed of the block C is upwards and is 25 m/s.

Given speeds are absolute speeds of pulleys and blocks.
Let us say the absolute speed of pulley p1 = v downwards.
Given absolute speed of pulley P2 = 10 m/s upwards.
Let the block C move upwards with speed = v2 m/s.

Speed of Block A Relative to the Pulley P1:    20 - v
Speed of block B wrt  pulley P1 :   10 + v

    The two relative speeds of A and B must be equal as the strings are tight and inextensible. 

            20 - v = 10 + v
            v = 5 m/s.  downwards.

Now pulley P2:
  relative velocity of pulley P1 wrt pulley P2 = 5 m/s + 10 m/s = 15 m/s
  relative velocity of block C wrt pulley P2 :   v2 - 10 m/s.

     These are equal. as the connecting string is tight.
              so v2 - 10 = 15 m/s.
                   v2 = 25 m/s.   speed of C upwards.
Done.