energy 問題

A bullet of mass 0.1 kg is shot into the wall and is stopped in 10 cm
If the resistive force acting on the bullet by the wall is 9800N
Estimate the speed of the bullet
Ans
K.E. lost by bullet=W.D. against resisting force
0.5*0.1*v^2=9800*0.1
v=140ms-1
如果我這樣做有問題嗎
Negative workdone by resisting force = lose in KE
-9800*0.1 = 0 - 0.5*0.1*v^2
v= 140
這樣做可以嗎 thx