posted by Nilan .
a bullet with a mass of 5 g and a speed of 600 m/s penetrates a tree to a depth of 4 cm. Assume that a constant frictional force stops the bullet.
Calculate the magnitude of this frictional force.
I used KE = 1/2 mv^2 = 900 J
then F (friction ) = W/d = 22500 N
BUT I don't know if it's positive or negative. please help. I just don't get it. Thank you.