Explanation:

When a body is thrown vertically upward with some initial velocity, it will reach a certain height and then fall back to the ground. At the greatest height, the body will have zero velocity, which means it will stop for a moment before falling back.

Let's look at the reasons why the final velocity of the body will be zero when it reaches the greatest height.

Gravity is the only force acting on the body:

When a body is thrown vertically upward, the only force acting on it is gravity. Gravity pulls the body towards the earth with a constant acceleration of 9.8 m/s^2. As the body moves upward, the gravity acts in the opposite direction to the motion of the body, which causes the velocity of the body to decrease gradually until it stops at the greatest height.

At the greatest height, the velocity of the body becomes zero:

As the body moves upward, its velocity decreases due to the force of gravity. At the greatest height, the body stops for a moment and then starts to fall back to the ground. This happens because the upward velocity of the body becomes zero, and the gravity starts pulling it back to the ground.

The body has reached its maximum potential energy:

When the body is at the greatest height, it has reached its maximum potential energy. Potential energy is the energy possessed by an object due to its position or state. At the greatest height, the body has the most potential energy because it is furthest away from the earth's surface. As the body falls back to the ground, its potential energy is converted into kinetic energy, which is the energy possessed by an object due to its motion.

Conclusion:

In conclusion, the final velocity of a body thrown vertically upward will be zero at the greatest height because gravity is the only force acting on the body, the velocity of the body becomes zero at the greatest height, and the body has reached its maximum potential energy.