1.What is the acceleration of a projectile when instantaneous vertical velocity is zero at the top of its trajectory?

The acceleration is always -g, in the downward direction if distance is measured positive upwards.

it depends... when you talk about acceleration of object going downward, its -g... when acceleration going up, example an object is thrown, i think its 0.

ax=o; ay= -g

To determine the acceleration of a projectile when the instantaneous vertical velocity is zero at the top of its trajectory, we need to understand the behavior of projectiles.

A projectile follows a curved path under the influence of gravity. Its motion can be divided into horizontal and vertical components. The horizontal component remains constant, assuming no external forces are acting on the projectile. However, the vertical component is affected by the acceleration due to gravity.

At the top of its trajectory, the vertical velocity becomes zero for an instant before the projectile starts descending. Since the projectile is momentarily at rest in the vertical direction, the only force acting on it is gravity, causing its velocity to decrease until it reaches zero.

The acceleration due to gravity always acts vertically downward and has a constant value of approximately 9.8 meters per second squared (m/s^2) on the surface of the Earth. This means that at the top of the projectile's trajectory, the acceleration is equal to the acceleration due to gravity but in the opposite direction.

Therefore, the acceleration of a projectile when the instantaneous vertical velocity is zero at the top of its trajectory is approximately -9.8 m/s^2, directed downward.