A bullet enters a thick wooden block with speed u and is uniformly retarded. After penetrating 3 cm, its speed becomes u/2. How much farther will it penetrate before stopping?
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A bullet enters a thick wooden block with speed u and is uniformly retarded. After penetrating 3 cm, its speed becomes u/2. How much farther will it penetrate before stopping?

G1 Physics · Accepted Answer

Using v² = u² + 2as, after 3 cm: u²/4 = u² + 2a(3). So 2a(3) = −3u²/4. For the remaining distance d, starting with speed u/2: 0 = u²/4 + 2ad. Divide the second relation by the first: d/3 = (u²/4)/(3u²/4) = 1/3. Hence d=1 cm.