Answer
The work done is 72,000 foot-pounds.
Work Step by Step
Since the crane is vertically lifting the boulder, the angle between the force and direction of motion is $0{}^\circ $.
Now it is known,
$\mathbf{W}=\left\| \mathbf{F} \right\|\left\| \mathbf{AB} \right\|\cos \theta $ …… (1)
As it is given that,
$\left\| \mathbf{F} \right\|=6000,\left\| \mathbf{AB} \right\|=12$ and $\theta =0{}^\circ $ …… (2)
Therefore, use (2) in (1).
So,
$\begin{align}
& \mathbf{W}=\left( 6000 \right)\left( 12 \right)\cos \left( 0{}^\circ \right) \\
& \mathbf{W}=72000
\end{align}$
