Gravitational force:
When we throw a ball into the air, it falls back due to the force of gravity. Gravity is a natural force that exists between any two objects with mass. It is responsible for the attraction of objects towards the center of the Earth. The force of gravity pulls objects downwards, causing them to fall.

Explanation:
- When we throw a ball into the air, we apply a force to it using our muscles (muscular force). This force propels the ball into the air and gives it an initial velocity.
- However, once the ball leaves our hand, the only force acting on it is the force of gravity. Gravity pulls the ball downwards towards the Earth.
- The force of gravity is always present and acts on all objects with mass. It is responsible for keeping us grounded on the Earth's surface and for the motion of objects falling towards the ground.
- As the ball moves higher into the air, gravity gradually slows down its upward velocity until it reaches its maximum height. At this point, the ball momentarily stops moving upwards before gravity starts to pull it back downwards.
- As the ball falls back towards the ground, gravity accelerates it, increasing its speed until it reaches the ground.
- The force of gravity acts vertically downwards, which is why the ball falls straight back towards the ground instead of moving in a different direction.
- The force of gravity is a fundamental force of nature that operates on a large scale, affecting not only objects on the Earth but also celestial bodies in space.

Conclusion:
The force responsible for the ball falling back when thrown into the air is the gravitational force. While we initially apply a muscular force to throw the ball, once it is in the air, the only force acting on it is gravity. Gravity pulls the ball downwards, causing it to fall back towards the ground.