Question

A bullet travels straight through a piece of wood of thickness 30mm. The change in the kinetic energy of the bullet is 1.4kJ. Calculate the average force exerted by the wood on the bullet.

Answer 1

According to the work-energy theorem, the net work done by the forces on an object equals the change in its kinetic energy:

$$W = \Delta K$$

On the other hand, by defiition, the work is:

$$W = Fs$$

where $F$ is the average force exerted by the wood on the bullet, and $s = 30mm = 0.03m$ is the distance travelled inside the wood.

Since in our case the change in bullet's kinetic energy is $\Delta K = 1400J$, obtain the following expression for the force:

$$Fs = \Delta K\\ F = \dfrac{\Delta K}{s}\\ F = \dfrac{1400J}{0.03m} \approx 46667N$$

Answer. 46667 N.