A bullet enters the first of several identical wooden planks, each of equal thickness, with a speed of 100 m/s and emerges from the first plank with a speed of 80 m/s. The retarding force exerted by the wood is constant and the same in every plank. The minimum number of such planks required to stop the bullet completely is:
Options

Question

A bullet enters the first of several identical wooden planks, each of equal thickness, with a speed of 100 m/s and emerges from the first plank with a speed of 80 m/s. The retarding force exerted by the wood is constant and the same in every plank. The minimum number of such planks required to stop the bullet completely is:

Options

G1 Physics · Accepted Answer

For each identical plank, the decrease in v^2 is constant. Across the first plank: delta(v^2) = 100^2 - 80^2 = 3600. After n planks, v_n^2 = 10000 - 3600n. For complete stopping, n >= 10000/3600 = 2.78. Therefore the bullet stops inside the 3rd plank. Minimum number = 3. Hence option B is correct.