In my opinion problem 2.49 from Serway book (see below)
is poorly formulated. Why do they distract student with "the
driver of the car?" He is only an observer. Why does he
"slam on the brake?"
On the other hand it is a good illustration for the topic we
discussed several days ago (the sign of acceleration). Is
the acceleration of the truck positive or negative? It depends
on the orientation of the x axis. The acceleration is negative
if the x axis points in the same direction in which the truck
is moving but it would be positive if the direction of the
x axis were reversed.
Let me type the problem for those who do not have that
textbook. The end of the book answer is correct, only if
one assumes that the truck was stopped (the end of the
skid mark) suddenly at the moment of the impact, like
if hitting a massive building, not a small car.
"The driver of a car slams on the brakes as a truck
approaches the car head-on while slowing uniformly
with acceleration -5.6 m/s^2. The driver of the car is
frozen with horror for 4.2 s while the truck makes
skid marks 62.4 m long. With what speed does the
truck strike the car?"
Ludwik Kowalski