when we throw an object upwards why the time of descent is greater rhan time of ascent?
The object's descent is just a time-reversed ascent, because total energy is conserved.
What specific situation are you considering? In general, the two times will be about the same, with any difference being caused by air resistance acting differently on the ascent and the descent.
in case we consider air resistance
Don't you think it would have been a good idea to tell us that initially?
The speed coming down will be less than the speed going up because the potential energy will be the same at the same height but the total energy will be less because of energy lost to air resitance. So kinetic energy will be less at the same height coming down than going up.
Even then, the details matter. A dense sphere (think cannonball) with an initial speed of a few tens of meters per second will not be measurably affected by air resistance - and this is also the problem you'll most often encountered in introductory physics classes. On the other hand, a spin-stabilized projectile that tumbles on the the way down, or a projectile whose initial velocity exceeds its terminal velocity in air will behave very differently on ascent and descent.
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