A bullet of mass 10 g moving horizontally with a velocity of 400 ms/s strikes a wood block of mass 2 kg which is suspended by light inextensible string of length 5 m. As a result, the centre of gravity of the block found to rise a vertical distance of 10 cm. The speed of the bullet after it emerges out horizontally from the block will be : 

120 m/s

Conservation of Linear Momentum : m x v = m' x v ' + m'' x v''

m = m' = 10 x 10^-3 kg ; v = 400 m/s ; m'' = 2 kg ; v'' = \(\sqrt{2gh}\)

⇒ v = 120 m/s