Discussion
Suppose a person fires a bullet of mass 100.0 g with a speed of 100 m/s on a target of plywood of thickness 20 cm. The bullet emerges from the target with only 20% of its initial kinetic energy. Then the emergent speed of the bullet is:
Answer: B
The kinetic energy is proportional to the square of the speed. If the bullet keeps 20 percent of its kinetic energy, then \(v^2 = 0.2 \times 100^2 = 2000\), so
\[ v = \sqrt{2000} = 44.72\ \text{m/s} \]The mass and the thickness of the plywood are not needed. A common slip is to take 20 percent of the speed, giving 20 m/s, or to use 0.2 times the energy without the square root; none of the other options, 34.70, 38.78 and 40.74 m/s, corresponds to any fraction of the energy that the question gives.
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