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The block has a mass of 20kg. A 50g rifle bullet is fired into the block, penetrating it by 10cm and causing it to swing upwards, increasing the height by 20cm.

a) Calculate the velocity of the bullet as it enters the wooden block. Express your answer in Km/h

Ep = 20*9.8*0.2 = 39.2 J

Ex = 0.5mv^2

39.2 = 0.5*20*v^2

v = 2m/s

m1v1 + m2v2 = (m1 + m2)*v'

0.05*v1 = (20.05)*2

v1 = 802m/s = 2887.2 km/h

b) Calculate the frictional force between the bullet and the wooden block

Wf = Ff * d

39.2 J = Ff * 0.1

Ff = 392 N

I'm not sure i was allowed to use the m1v1 + m2v2 = (m1 + m2)*v' in a).

Can anyone confirm this?