A hunter spies a monkey in a tree, takes aim, and fires. At the moment the bullet leaves the gun the monkey lets go of the tree branch and drops straight down. How should the hunter aim to hit the monkey?

1. Aim directly at the monkey
2. Aim high (over the monkey's head)
3. Aim low (below the monkey)

The monkey starts a vertical distance h above the hunter, and the horizontal distance between the monkey and hunter is d. The origin is the hunter.

The horizontal position of the bullet is:

x_b = v_oxt

When x_b = d, t = d/v_ox

The vertical position of the bullet is:

y_b = v_oyt - ½ gt²

Plugging in t = d/v_ox for the first t gives:

y_b = d v_oy/v_ox - ½ gt²

Compare this to the vertical position of the monkey:

y_m = h - ½ gt²

If the hunter aims the gun at the monkey to begin with:

tan(θ) = h/d = v_oy/v_ox

So, d v_oy/v_ox = h

In this case, when the bullet has traveled a distance d horizontally, its vertical position will be:

y_b = h - ½ gt²

That exactly matches the monkey's position, so the bullet will hit the monkey when the hunter aims the rifle at the monkey.