Physics · Motion In A Straight Line · NEET
It depends on which direction you call positive, not on whether the object is rising or falling. If you take UP as positive (the NCERT choice), then g = -9.8 m/s^2 for the whole motion. If you take DOWN as positive, then g = +9.8 m/s^2. The physics is the same; only the labels change. Pick one convention and keep it for the whole problem.
No. This is the most common mistake. Acceleration due to gravity is constant in both magnitude and direction for the entire flight, up and down. If up is positive, then g = -10 m/s^2 while rising, at the top, and while falling. What changes sign is the velocity, not the acceleration. At the top velocity = 0, then it becomes negative as the ball moves down.
Because sign shows direction, not speed. Taking up as positive, the ball moves up (positive velocity) but slows down. Slowing down while moving in the positive direction needs an acceleration pointing the other way, i.e. downward, so a = -g = -10 m/s^2. Gravity is the reason the ball slows, stops, and returns.
You are free to choose, but taking DOWN as positive makes the arithmetic cleanest because everything (velocity, displacement, g) is positive. Then u = 0, a = +g, and the fall distance h is positive. Just remember: if you later have a case where the object also moves up, switch to up-positive to avoid sign errors.
Displacement takes the sign of the direction of the net change from the starting point. For a ball thrown up from a bridge that lands in the water below, the final position is BELOW the start, so its displacement is negative (e.g. -64 m). The height of the bridge is the magnitude, 64 m. Always report distances/heights as positive magnitudes even when the signed displacement is negative.
A ball is thrown vertically downward with a velocity of 20 m/s from the top of a tower. It hits the ground after some time with a velocity of 80 m/s. The height of the tower is (g = 10 m/s^2):
A ball is thrown vertically upward with a velocity of 4 m/s from a bridge. The ball strikes the water surface after 4 s. The height of the bridge above the water surface is (g = 10 m/s^2):
Try the real previous-year questions from this chapter — each with the answer and a full solution.
NCERT chooses the upward direction as positive. Since gravity acts downward, acceleration due to gravity is negative: a = -g = -9.8 m/s^2. This convention is used for both the rising and falling parts of the motion.
No. The magnitude (9.8 or 10 m/s^2) and the direction (always downward) stay the same throughout. Only the velocity changes: it decreases, becomes zero at the top, then increases in the downward direction.
You can take g = +9.8 m/s^2 if you define DOWN as positive, which is convenient for pure dropping problems. But you must then keep velocity and displacement signs consistent with that same choice. Never mix conventions inside one problem.
Velocity = 0 at the highest point, but acceleration = g (downward) = -10 m/s^2 (up positive). Acceleration is never zero in free fall; that is why the ball does not stay at the top and starts to fall back.
Choosing and sticking to one sign convention prevents the most common numerical errors in kinematics. NEET regularly sets balls thrown up/down from towers and bridges (2020, 2023), and a wrong sign on g or on the launch velocity gives one of the trap options.
