When an object is thrown straight up into the air, it experiences a vertical motion governed by the force of gravity. As the object rises, its velocity decreases until it reaches a maximum height, also known as the top of its arc. At this point, the object's velocity is momentarily zero.
This phenomenon can be understood by considering the forces acting on the object. Initially, when the object is thrown upward, it experiences an upward force due to the initial throw. At the same time, the force of gravity acts downward, causing the object to decelerate. As the object moves higher, gravity continues to slow it down until it eventually brings it to a momentary halt.
At the top of its arc, the object's velocity is zero because the opposing forces of gravity and the initial throw balance each other out. The force of gravity acts in the opposite direction of the initial throw, gradually reducing the object's upward velocity. Eventually, the object's upward velocity decreases to zero, and gravity becomes the dominant force, causing the object to reverse its direction and fall back down.
It's important to note that although the object's velocity is momentarily zero at the top of its arc, its acceleration due to gravity is still present. The acceleration due to gravity always acts downward, causing the object to accelerate downward as it descends from the top of its arc.