When an object is thrown straight up into the air, it undergoes a process of deceleration due to the force of gravity acting upon it. This deceleration causes the object's upward velocity to decrease until it reaches zero at the highest point of its trajectory. After reaching its peak, the object then starts to accelerate downward under the influence of gravity.
The reason the object comes back down at a slower speed than when it was thrown upward is because of the effects of gravity and air resistance.
Gravity, which is constantly pulling the object downward, causes it to accelerate as it falls back towards the Earth. However, air resistance acts in the opposite direction, opposing the motion and slowing down the object's descent.
When the object is initially thrown upward, it encounters both the force of gravity pulling it downward and air resistance acting against its upward motion. The combined effect of gravity and air resistance causes the object to slow down as it rises, resulting in a decrease in its upward velocity.
As the object reaches its highest point, the upward velocity decreases to zero, and then gravity begins to dominate the motion, causing the object to accelerate downward. However, during the downward motion, the air resistance continues to act against the object's motion, slowing down its descent.
Due to the opposing forces of gravity and air resistance, the object returns to the ground at a slower speed than when it was initially thrown upward.