the initial speed of 15g bullet is 600 m/s. it penetrates 10cm into the stationary target post before it stops.?

KE = Work

1/2 mv² = F*d

F = 1/2 mv² /d

Using KMS system:

= 1/2 [15g*(1kg/1000g)] (600m/s)² /[10cm*(1m/100cm)]

= 27,000 N

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If you use CGS, the answer would be in dynes

Take care with units.

15g = 0.015 kg

10 cm = 0.1 m

The initial kinetic energy of the bullet: ½mv²

= ½ * 0.015 * 600²

= 2700 joules

Work done stopping bullet = kinetic energy lost = force * distance

Average force = 2700 / 0.1 = 27000 newtons

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If you prefer a kinematics based solution:

v² = u² + 2as

0 = 600² + 2 * a * 0.1

a = -1800000 m/s²

Newton's second law gives us force = mass * acceleration

force = 0.015 * -1800000 = -27000 N

{The minus sign is just saying that the force is acting in the opposite direction to the bullet motion.}