A bullet moving at a speed of 154 m/s passes through a plank of wood. After passing through the plank, its speed is 149 m/s. Another bullet, of the same mass and size but moving at 91 m/s, passes through an identical plank. What will this second bullet's speed be after passing through the plank? Assume that the resistance offered by the plank is independent of the speed of the bullet.

To determine the second bullet's speed after passing through the plank, we can use the principle of conservation of momentum. According to this principle, the total momentum before an event is equal to the total momentum after the event.

In this case, we have two scenarios:

1. The first bullet has a speed of 154 m/s before passing through the plank and a speed of 149 m/s after passing through the plank.
2. The second bullet has a speed of 91 m/s before passing through the identical plank.

Since both bullets have the same mass and size, their initial momentum is the same. Therefore, we can set up the following equation:

Initial momentum of the first bullet = Initial momentum of the second bullet
(mass of the bullet) x (initial velocity of the first bullet) = (mass of the bullet) x (initial velocity of the second bullet)

Since the mass of the bullet cancels out, we can rewrite the equation as:

(initial velocity of the first bullet) = (initial velocity of the second bullet)

Now, we can substitute the given values:

154 m/s = 91 m/s

Thus, the second bullet's speed after passing through the plank will still be 91 m/s.