When will a ball undergo negative acceleration if it is thrown vertically upwards?

When the ball is in flight*, its acceleration is CONSTANTLY of magnitude 9.8m/s² directed downward. If you choose a reference frame in which "up" is the positive direction, then the acceleration is constantly negative (constantly −9.8m/s²) because of its constant downward direction.

The acceleration does NOT change as the ball moves through the air*. It doesn't depend on the ball's position, speed, or direction at any given moment. Having a constant vertical acceleration of −9.8m/s² just means that the ball's vertical speed changes by −9.8 m/sec every second. In other words, as each second passes, you subtract "9.8" from the ball's vertical speed. That means if the ball's speed is initially positive (upward), then eventually (after subtracting 9.8 a few times) it will become negative (downward).

I know it's confusing, because there are certain (other) physics problems in which they say the acceleration is "negative" if the object is slowing down, and "positive" if the object is speeding up. But that happens because of the way the coordinate system is defined for the particular problem. Very often, people DEFINE the coordinates so that "positive" stands for "the initial direction of motion." This convention is confusing in the case of things moving through the air, because objects could be moving in any direction.

So, a good practice is, when you're analyzing a motion problem, make sure you have a clear picture of which direction is considered "positive" and which "negative". Sometimes they'll tell you that (or you can infer it) in the problem's description; other times you will need to make your own decision (your choice won't affect the final answer). But in either case, make sure you maintain that convention throughout the entire analysis.

*(Strictly speaking, the ball's acceleration is constant only if (a) air resistance is negligible; and (b) the ball stays fairly near the earth's surface.)

"Negative" acceleration just means that the ball is accelerating in a negative direction. If its already moving in that direction, then it's going faster and faster. If its already moving in a positive direction, then its slowing down and will eventually start moving in the negative direction.

So it all depends on which direction you call "negative". Pick a direction, call it negative, and you will get your answer.

If you pick "up" as negative, then the ball will never have negative acceleration (except while your hand is pushing it upwards, and when it finally hits the ground). It is always accelerating downwards, which is the direction you picked as positive.

If you pick "down" as negative, then the ball will always have negative acceleration. It is always accelerating downwards, which is the direction you picked as negative.

permit t be the time they could desire to fulfill. then the dropped ball has the cost vA = g * t at a similar time as the increasing ball has the cost vB = a million/2 vA = a million/2 gt Ball A fell f = a million/2 g t^2 Ball B rose r = a million/4 gt^2 the heigth is h = r + f = 3/4 gt^2 = 3 * r So h = 3 * r ---> r = a million/3 * h

no.

a ball (or any object) thrown upwards experiences constant acceleration: 9.8 m/s² (on Earth).