#### Answer

The magnitude of the friction force on the truck is $1.01\times 10^4~N$.

#### Work Step by Step

Since the truck does not move, the net force on the truck is zero. Therefore, the friction force must be equal in magnitude to component of the truck's weight which is directed down the slope. We can find the magnitude of the friction force.
$F_f = mg~sin(\theta)$
$F_f = (4000~kg)(9.80~m/s^2)~sin(15^{\circ})$
$F_f = 1.01\times 10^4~N$
The magnitude of the friction force on the truck is $1.01\times 10^4~N$.