Question 6
When an object is travelling vertically upwards, and the upward direction is taken as positive, what can be said about the acceleration due to gravity?
The right answer is the one marked above.
The acceleration takes a negative value; the velocity decreases. The force of gravity always acts towards the earth — that is, downwards. For an object travelling upwards, it opposes the direction of motion.
So for vertical upward motion the value taken for the acceleration due to gravity is −9.8 m s-2. The velocity drops by 9.8 m s-1 every second, reaching zero at the maximum height.
One important point: the acceleration does not become zero. At the top it is the velocity that is zero — gravity keeps acting, so the acceleration stays at 9.8 m s-2 downwards, and that is precisely why the object then starts to fall.