an atwood machine is set up by suspending two blocks connected by a string of negligible mass over a pulley, as shown in the figure above. the pulley has negligible mass but there is friction as it rotates. the system is released from rest, and after 1.0 s the speed of the 3 kg block is 1.8 m/s. which of the following is the best estimate of the external frictional force acting on the two-block system?
a. 0,4 N
b. 0,6 N
c. 1,0 N
d. 1,3 N