## Precalculus (6th Edition) Blitzer

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}