I have learned that attraction between the bodies increases when it's mass increases ,if the population increases day by day and hence it can increase the mass of earth which causes attraction between the earth and moon more intense, so moon should ultimately collide with earth ?
- Conservation of mass: As a whole, mass (atoms) on Earth is roughly conserved (in fact, the Earth loses a lot of hydrogen gas every year, but it can be considered negligible). Manufacturing new objects requires natural resources to be extracted from somewhere else.
- The Moon will not collide with Earth because it is travelling in approximately a circular orbit around Earth, which requires a centripetal force that is provided by gravitational force. In fact, due to tidal effects, the Moon is drifting further from Earth at approximately 3.8 cm per year.
The population is made out of atoms, like carbon and oxygen, which have to be extracted from somewhere else on Earth, so the total mass of Earth does not change as population grows.
The mass of the Earth does not change as the population changes, as V.F. pointed out. Of course, the masses of the Earth and Moon are not constants of nature so both are subject to change. However, the actual changes in the masses of the Earth are Moon (due to ordinary processes such as meteor impacts) are far too small to have any effect on the dynamics of the Moon's orbit.
A much larger and more interesting effect is the tidal interaction between the Earth and Moon, which causes the Earth's rotational velocity to slowly decrease (the day has increased by 1.7 milliseconds over the last century) and the Moon to slowly recede from the Earth (about 4 centimeters per year-- much less than the escape velocity at the Moon's current distance, so the Moon will not escape from the Earth).