alleryStart learning →A train travelling at -20.0 km/h slows to a stop in 10.0 s. What is its average acceleration in SI units?
A-0.556 m/s^2
B-2.00 m/s^2
C+0.556 m/s^2
D+2.00 m/s^2
Answer & Solution
Correct answer: C. +0.556 m/s^2
1. Identify the knowns: the initial velocity is -20.0 km/h and the final velocity is 0.
2. The change in velocity is 0 - (-20.0 km/h) = +20.0 km/h, which is positive.
3. Convert to SI units: 20.0 km/h x (1000 m / 1 km) x (1 h / 3600 s) = 5.56 m/s.
4. Average acceleration is the change in velocity divided by the elapsed time.
5. Substitute the values: (5.56 m/s) / (10.0 s) = +0.556 m/s^2.
6. The acceleration is positive even though the train is slowing, because the velocity was negative.
7. Assuming a slowing object must have a negative acceleration is the trap here.
_Source: OpenStax College Physics (CC BY 4.0), Ch 2 "Kinematics", section 2.4 Acceleration_
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