A bullet of mass 20 g enters a fixed wooden block with a speed of...

The initial kinetic energy of the bullet is given by $K_{i} = \dfrac{1}{2}mv^{2}$.

Substituting the given values, $m = 20 \text{ g} = 0.02 \text{ kg}$ and $v = 40 \text{ m s}^{-1}$:

$K_{i} = \dfrac{1}{2} \times 0.02 \times (40)^{2}$

$K_{i} = 0.01 \times 1600 = 16 \text{ J}$

The final kinetic energy of the bullet is $K_{f} = 0$ since it comes to rest.

According to the law of conservation of energy, the loss in kinetic energy is converted into the internal energy of the system.

Change in internal energy $\Delta U = K_{i} - K_{f} = 16 \text{ J} - 0 = 16 \text{ J}$.