easy m2 question

Q) A bullet of mass 25 g is fired directly at a fixed wooden block of thickness 4 cm and passes through it. When the bullet hits the block, it is travelling horizontally at 200 m s-1. The block exerts a constant resistive force of 8000 N on the bullet.

(a) Find the work done by the block on the bullet. (2 marks)
Answer: 8000*0.04= 320J

By using the Work-Energy principle,
(b) show that the bullet emerges from the block with speed 120 m s-1. (5 marks)

My working:
Change in ME= WD by external forces
(0.5)(0.025)(v^2-200^2)=320
then solved to find v=256.1

Can anyone tell me where I've gone wrong? thanks :biggrin: