In a ballistics demonstration, a police officer fires a bullet mass `50.0g` with speed `200ms^(-1)` on soft plywood of thickness 2.00cm. The bullet emerges only with `10%` of its initial kinetic energy. What is the emergent speed of the bullet ?

Explanation

Initial kinetic energy, `K_(i)=(1)/(2)xx(50)/(1000)xx200xx200 J=1000 J`
Final kinetic energy, `K_(f)=(10)/(100)xx1000 J=100 J`
If `v_(f)` is emergent speed of the bullet, then
`(1)/(2)xx(50)/(1000)xx v_(f)^(2)=100`
implies`v_(f)^(2)=4000 implies v_(f)=63.2 ms^(-1)`
Note that the speed is reduced by approximately 68% and not 90%