Question

19) Suppose that a car traveling to the east (+x direction) begins to slow down as it approaches a traffic
light. Which statement concerning its acceleration must be correct?
A) Its acceleration is in the -x direction.
B) Its acceleration is zero.
Its acceleration is decreasing in magnitude as the car slows down.
D) Its acceleration is in the +x direction.

Answer 1

Answer: C) Its acceleration is decreasing in magnitude as the car slows down.

Explanation:

According to the described situation the car's final velocity is less than its initial velocity, this means the car's velocity decreased as it slows down.

On the other hand, acceleration $a$ is defined as the variation of velocity in time:

$a=\frac{\Delta V}{\Delta t}$  

If the car's velocity decreases, its acceleration is negative.  Hence, the acceleration is decreasing in magnitude as well.