Final answer:
To determine how long an object is descending when projected vertically upwards, one must consider the total time the object spends in the air, which is governed by its vertical motion. The descent time can be calculated from the projectile's initial velocity and the change in altitude.
Step-by-step explanation:
The time an object takes to descend when projected vertically upwards is determined by its vertical motion. The period of descent is half of the total time the projectile spends in the air, assuming it lands back at the starting altitude. If the object lands below its starting altitude, the total time can be calculated using the initial vertical velocity and the distance it falls. For example, a projectile with an initial vertical velocity of 14.3 m/s falling 20.0 m below its starting altitude will spend 3.96 seconds in the air.
An application of this concept is free fall, where the object is subject to a uniform gravitational field with no initial horizontal velocity. Even if an object is thrown upwards, the equations of free fall still apply to both its ascent and descent phases. Using the principles of projectile motion and free fall, one can resolve the motion into vertical and horizontal components to calculate times and velocities for various points in the projectile's trajectory.