Question

A $30g$ bullet initially travelling at $120m/s$ penetrates $12cm$ into the wooden block. The average resistance exerted by the wooden block is:

• A. $1600N$
  
• B. $1800N$
  
• C. $2000N$
  
• D. $2200N$
  

Answer 1

The correct option is B $1800N$

Work done stopping bullet = change in kinetic energy of bullet.

Take care with units!
$30g=0.030kg$
$12cm=0.12m$

Initial $kineticenergy=0.5mv²=(0.5\ast 0.030kg\ast 120m/s²)=216joules$

Work done stopping bullet =$216J$

$Workdone=averageforce\ast distance$
$216=force\ast 0.12force=\left(216/0.12\right)=1800N$