When a rock is thrown straight up in the air, after it leaves the hand, the rock begins to slow down. This occurs because:
The rock slows down because of the force of gravity acting upon it. When a rock is thrown straight up in the air, it initially has an upward velocity due to the force exerted by the person throwing it. However, as soon as it leaves the hand, the only force acting upon it is gravity pulling it back down to the ground.
To understand why the rock slows down, we need to consider Newton's laws of motion. According to Newton's second law, the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. In this case, the net force acting on the rock is the force of gravity pulling it downward.
As the rock moves upward, the force of gravity acts in the opposite direction to its motion. This creates a deceleration, or a negative acceleration, on the rock. As a result, the rock gradually loses its upward velocity and comes to a stop at its highest point, known as the apex.
After reaching the apex, the force of gravity continues to act on the rock, but now in the same direction as its motion. This causes the rock to accelerate downward, gaining speed as it falls back to the ground. So, the rock slows down as it moves upward due to the opposing force of gravity.