A car moves towards the left and speeds up, with rightward taken as positive. Which description is correct?
ANegative acceleration, and the car is decelerating
BPositive acceleration, and the car is decelerating
CPositive acceleration, and the car is speeding up
DNegative acceleration, and the car is speeding up
Answer & Solution
Correct answer: D. Negative acceleration, and the car is speeding up
1. Rightward is positive, so leftward motion means a negative velocity.
2. The car speeds up, so its velocity grows more negative and the change in velocity points left.
3. Acceleration is in the direction of the change in velocity, so the acceleration is negative.
4. Deceleration means acceleration opposite to the velocity, which is not the case here.
5. Acceleration and velocity share the same direction, so the car is speeding up.
6. Calling it deceleration is the standard trap, because the acceleration carries a minus sign.
_Source: OpenStax College Physics (CC BY 4.0), Ch 2 "Kinematics", section 2.4 Acceleration_
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