Final answer:
The correct answer is (a) 9.8 m/s² downward because the acceleration due to Earth's gravity is a constant 9.8 m/s² downward, regardless of the object's movement direction.
Step-by-step explanation:
The acceleration of an object being thrown up on Earth is a result of Earth's gravitational pull. Despite the object's initial movement upward, the acceleration due to gravity always acts downward towards the center of Earth. As such, regardless of the object moving up or coming down, the acceleration due to gravity remains constant at an average of 9.8 m/s² downward.
Therefore, the correct answer is (a) 9.8 m/s² downward. When we define the upward direction as positive, the acceleration due to gravity is negative, which means the value of acceleration a in kinematic equations is -9.8 m/s².
Conversely, if downward is positive, then a is 9.8 m/s². In this context, we typically choose the upward direction as the positive axis for solving problems involving objects thrown upwards.
An example for further understanding: When a rock has an initial velocity of 15 m/s upward, from the moment it leaves the hand or device, it slows down at 9.8 m/s² until it reaches its peak height, where the velocity is zero before starting to fall back down with increasing speed, still at the same acceleration rate of 9.8 m/s², but in the opposite direction of its initial velocity.