A bullet enters a wooden block of thickness d with speed u and emerges from the other side with speed u/2. The resisting force offered by the wood is constant. An identical wooden block is placed immediately behind the first block. The distance travelled by the bullet inside the second block before coming to rest is:
Options

Question

A bullet enters a wooden block of thickness d with speed u and emerges from the other side with speed u/2. The resisting force offered by the wood is constant. An identical wooden block is placed immediately behind the first block. The distance travelled by the bullet inside the second block before coming to rest is:

Options

G1 Physics · Accepted Answer

Let the magnitude of retardation be a. In the first block: (u/2)^2 = u^2 - 2ad, so 2ad = 3u^2/4. In the second block, if penetration before stopping is x: 0 = u^2/4 - 2ax. Dividing gives x/d = (u^2/4)/(3u^2/4) = 1/3. Hence x = d/3 and option B is correct.