Question

Block A weights $4$N and blocks weight $8$N. The coefficient of kinetic friction is $0.25$ for all surfaces. Find the force F to slide B at a constant speed when (a) A rests on B and moves with it, (b) A is held at rest, and (c) A and B are connected by a light cord passing over a smooth pulley as shown in Figure, respectively.

Answer 1

frictional force,

$f_2=(8+4){\mu }_k=0.25\times 12=3N$

If block A and B move with constant speed, then $F$ should be equal to $f_2$

$F=f_2=3N$

ii) In this case;

$f_1={\mu }_k\left(4N\right)=0.25\times 4=1N$

$f_2=3N$

$F=f_1+f_2=4N$

iii) In this case, if B move with constant speed then A also move with constant speed but opposite direction.

$F=T+f_1+f_2=1+1+3=5N$