Question

A bullet with a mass of 4.26 g and a speed of 881 m/s penetrates a tree horizontally to a depth of 4.44 cm. Assume that a constant frictional force stops the bullet. Calculate the magnitude of this frictional force. Try energy considerations.

Answer 1

Let F be the magnitude of the frictional force. This force performs an amount of work W on the bullet such that

W = -Fx

where x is the distance over which F is acting. This is the only force acting on the bullet as it penetrates the tree. The work-energy theorem says the total work performed on a body is equal to the change in that body's kinetic energy, so we have

W = ∆K

-Fx = 0 - 1/2 mv²

where m is the body's mass and v is its speed.

Solve for F and plug in the given information:

F = mv²/(2x)

F = (0.00426 kg) (881 m/s)² / (2 (0.0444 m))

F = 37,234.8 N ≈ 37.2 kN