Question

Two blocks are connected by a string, as shown in figure. They are released from rest. The coefficient of kinetic friction between the upper block and the surface is 0.5. Assume that the pulley is massless and frictionless. Their common speed after they have moved a distance $5m$ will be: ($g=10m{s}^{-2}$)

• A. $10\text{m/s}$
• B. $5\text{m/s}$
• C. $2.5\text{m/s}$
• D. $25\text{m/s}$

Answer 1

The correct option is B $5\text{m/s}$


$f_k=0.5\left(20\right)$
$=10\text{N}$
Using conservation of energy
$K_i+U_i+W_f=K_f+U_f$
$0+0+f_{k}x\cos {180}^{\circ }=2\times \frac{1}{2}m{v}^2-mgx$
$10\left(5\right)(-1)=2\times \frac{1}{2}\left(2\right)(v{)}^2-2\times 10\times 5$
$-50=2v^2-100$
$v^2=25$
$v=5{\text{ms}}^{-1}$