At the highest point of the projectile's trajectory, the vertical component of its velocity is momentarily zero. This means that the projectile has reached its peak height and is about to descend. At this point, the only force acting on the projectile is the force of gravity.
The acceleration due to gravity is a constant value near the Earth's surface and is typically denoted by the symbol "g." Its approximate value is 9.8 m/s².
Since the projectile is momentarily at rest in the vertical direction, the acceleration due to gravity cancels out the upward velocity component until the projectile begins to descend. Therefore, the magnitude of the acceleration at the highest point of the projectile's trajectory is equal to the acceleration due to gravity, which is approximately 9.8 m/s².
So, the magnitude of the acceleration at the highest point is 9.8 m/s².