Answer to Question #253947 in Physics for Kristine

Question

A bullet with a mass of 5.00 𝑔𝑟𝑎𝑚𝑠 and a speed of 600 𝑚/𝑠 penetrates a tree to a depth of 4.00 cm. Find the average friction force that stops the bullet. Assuming that the frictional force is constant, determine how much time elapsed between the moment the bullet entered the tree and the moment it stopped.

Answer 1

Apply work-energy theorem:

$$W=KE,\\\space\\ Fl=\frac12mv^2,\\\space\\ F=\frac{mv^2}{2l}=22500\text{ N}.$$

The time is

$$t=v/a=v/(F/m)=vm/F=1.33·10^{-4}\text{ s}.$$