Posted by **Nilan** on Sunday, February 20, 2011 at 6:09pm.

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.

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