A 2.40-{\rm g} bullet embeds itself in a 1.40gblock, which is attached to a spring of force constant

A 2.40g bullet embeds itself in a 1.40kg block, which is attached to a spring of force constant 750 N/m.

a) If the maximum compression of the spring is 5.60 cm, find the initial speed of the bullet.
b)Find the time for the bullet-block system to come to rest.

a) Energy in spring = 1/2 . k . x^2 = 0.5 . 750 . 0.056^2 = 1.176 J

This starts as KE so the speed was

v = √ 2 E / m = √ 2 . 1.176 / 1.4024 = 1.68 m/s

Speed of the bullet = 1.68 . 1.4024 / 0.0024 = 980 m/s

b) T = 2π √ m/k = 2π √ 1.4024 /750 = 0.27 s

Time to come to a stop = T/4 = 0.068 s