Question

A bullet of mass $200\, g$ having initial kinetic energy $90\, J$ is shot inside a long swimming pool as shown in the figure. If it's kinetic energy reduces to $40\, J$ within $1\, s$, the minimum length of the pool, the bullet has a to travel so that it completely comes to rest is

• A. $45 \,m$
• B. $90 \,m$
• C. $125\,m$
• D. $25\, m$

Answer 1

Answer: (A)

Using $mv =\sqrt{2 mk }$
$ u =\frac{1}{0.2} \sqrt{2 \times 0.2 \times 90}=30 \,m / s $
$ v =\frac{1}{0.2} \sqrt{2 \times 0.2 \times 40}=20 \, m / s$
$ a =\frac{20-30}{1}=-10 \, m / s ^2 $
$ s =\frac{- u ^2}{2 a }=45 \,m$