Question

If the upward direction is positive, which situation involves negative velocity and positive acceleration?
A. An elevator moving upward at a constant speed.
B. An elevator speeding up as it moves downward.
C. An elevator speeding up as it moves upward.
D. An elevator slows down as it moves downward.

Answer 1

Hint: If the velocity is negative, then the elevation is going in the opposite direction. And also positive acceleration means there is a change in velocity. If the velocity is constant, then there is no acceleration involved.

Complete answer:
As velocity is the rate of change of displacement, when the elevation is going down the displacement is decreasing thus the velocity is negative. The rate of change in velocity, however, is positive because the acceleration is positive. So, while the velocity is increasing, its magnitude (speed) is decreasing over time.A negative velocity object is one that moves in the opposite direction. When an item slows down, its acceleration vector is in the opposite direction of its motion (in this case, a positive acceleration).

Positive acceleration refers to the acceleration of a bus or car as it speeds up. It is possible to claim that it is speeding up in the direction of travel. Negative acceleration= In the example above, when we apply the brake, the car's speed decreases. Then it accelerates backwards.
The item is slowing down if the velocity and acceleration have opposing signs (opposite directions). A ball thrown upward, for example, has a positive velocity and a negative acceleration as it rises.

The combined system of you and the elevator is subjected to two forces: gravity and cable tension. Consider the normal force exerted by the elevator on you: If the elevator is at rest or travelling at a steady speed, \[N = mg\] ; if the elevator is accelerating downhill, \[N = mg - ma\] .

Hence, the correct option is D.

Note: When travelling up in an elevator, there are two instances when you don't move at a steady speed: when speeding up and when slowing down as you approach the higher floor. If the elevator is slowing down, keep in mind that the acceleration vector is opposite the velocity vector.