Homework Help: physics

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.

physics - bobpursley, Sunday, February 20, 2011 at 6:17pm
force*distance= 1/2 m v^2 (bullet)

force*.04m= 1/2 * .005kg * 600^2 m^2/s^2

solve for force.
physics - Nilan, Sunday, February 20, 2011 at 6:40pm
So F = 22500

Am I right in thinking it is POSITIVE?

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