Aprojectile is launched at a diagonal angle and is in the air for 17.5 seconds. how long does it take the projectile to reach its apex?

Explanation:

For a projectile the total time it stays in the air is called the flight time. The path traced during a projectile motion is parabolic with symmetry of time and distance.

Given that the projectile having a flight time of 17.5 seconds, will reach the top point in half of the flight time taken by it:

So,

is the time by the end of which the projectile will be at its apex having the vertical component of velocity as zero.

Do note that during a projectile analysis we neglect the air resistance due to which we have a constant horizontal component of the velocity and the vertical component of the velocity varies under the influence of gravity. The vertical velocity at the top becomes zero.