Question

Three blocks A, B and C are kept as shown in the figure. The coefficient of friction between A and B is 0.2, B and C is 0.1, C and between C and ground is 0.0. The masses of A, B and C are 3 kg, 2kg and 1 kg respectively, A is given a horizontal velocity 2 m/s. A, B and C always remain in contact. Find (in J) the magnitude of total work done by friction.

$K_f-K_i=W_{friction}$ (K= Kinetic energy)

• A. 3 J
• B. -3 J
• C. 6 J
• D. -6 J

Answer 1

The correct option is B

-3 J

Consider all the blocks as one system. As there is no external force in the horizontal direction,

$\Rightarrow$ P (momentum) = constant

$P_i=3\times 2=6kgm/s$

Let's assume the system moves with V after some time $\Rightarrow$$P_f=6\times Vkgm/s$

From conservation of linear momentum,

6V = 6

V = 1 m/s

Now we know work. Energy theorem,

$K_f-K_i=W_{friction}$

$3\times 1-\frac{1}{2}\times 3\times 4=W_{friction}$

$W_{friction}=-3J$