A bear can be knocked down by hitting one of the following bullets of same momentum:

Explanation

The key to understanding the problem lies in the concept of change in momentum. A rubber bullet will bounce back upon impact, changing its direction and effectively doubling the change in momentum imparted to the bear. The formula for change in momentum is Δp = mv - (-mv) = 2mv, where the negative sign indicates a reversal of direction. This larger change in momentum results in a greater force applied to the bear. In contrast, bullets made of lead, steel, or gold will penetrate or embed themselves in the bear, resulting in a smaller change in momentum, as they do not reverse direction and thus have Δp = mv - 0 = mv. Therefore, a rubber bullet is more likely to knock down the bear due to the greater force from the double change in momentum.