Given data

• Mass of box: $m = 50\,\text{kg}$
• Coefficient of kinetic friction: $\mu_k = 0.3$
• Surface is horizontal

Formula for kinetic friction On a horizontal surface, the kinetic friction is $f_k = \mu_k N$ where $N$ is the normal reaction.

Find normal reaction Since the surface is horizontal and no other vertical forces are mentioned, $N = mg$ Taking $g = 9.8\,\text{m/s}^2$, $N = 50 \times 9.8 = 490\,\text{N}$

Calculate kinetic friction $f_k = 0.3 \times 490 = 147\,\text{N}$

Match with options

• A: $1470\,\text{N}$
• B: $147\,\text{N}$
• C: $1.47\,\text{N}$
• D: $14.7\,\text{N}$

Therefore, the correct option is B.

Verification with stored answer Stored correct answer: B

My derived answer also gives B.