In a ballistics demonstration a , police officer fires a bullet of mass 50 g with speed `200ms^(-1)` on soft plwood of thickness 2 cm . The bullet emerges with only 10 % of its initital kinetic energy . What is the emergent speed of the bullet ?

Explanation

The initial kinetic energy of the bullet is `mv^(2) //2 = 1000 J` . It has a final kinetic energy of 0.1 `xx 1000 = 100` J. If `v_(f)` is the emergent speed of the bullet,
`(1)/(2) mv_(f)^(2) = 100 J`
`v_(f) = (sqrt(2 xx 100 J))/0.05 kg = 63 . 2 ms^(-1)`
The speed is reduced by approximately `68%` (not `90%`) .