EXPLANATION:
The first and foremost equation one must know is the equation relating the force on a body, its mass and acceleration.
It is as follows:
F=ma
Another equation relating the initial and final velocity is as follows:
V= u+at
HERE:
V=is the final velocity.
u= is the initial velocity.
a=is the acceleration.
t=is the time taken.
Let us first find out the deceleration of the truck:
We have:
v=u+at
with:
v=0
u=20
t=120
Thus we have :
0=20+a(120)
a=-1/6m/s²