Final answer:
The acceleration of an object dropped from a height under gravity is approximately 9.8 m/s² downward but often simplified to 10 m/s² downward. The correct answer is (a) 10 m/s² downward.
Step-by-step explanation:
An object that drops from a height and is subject to Earth's gravity undergoes an acceleration due to gravity, which is approximately 9.8 m/s² downward. When the object starts at rest and ends up moving at 20 m/s just before it hits the ground, the acceleration responsible for this change in velocity is indeed gravity. Therefore, the object's acceleration is 9.8 m/s² downward, which is rounded to 10 m/s² downward for simplicity in most physics problems. The answer to the student's question is (a) 10 m/s² downward. This constant acceleration applies to all freely falling objects, regardless of whether they are moving upwards or downwards or are at the top of their trajectory. As it falls, this acceleration value remains the same until it impacts the ground.