Importance of the universal law of gravitation

Newton's insight was unification, not invention — gravity between two objects was already suspected; his contribution was showing it is universal, acting between any two masses anywhere, from an apple and the Earth to the Earth and the Sun.

where: $F$ is the gravitational force of attraction (newton, N), $G = 6.7\times10^{-11}\ \text{N m}^2/\text{kg}^2$ is the universal gravitational constant, $m_1$ and $m_2$ are the two masses (kg), and $r$ is the distance between their centres (m). Four consequences follow directly, and each is a standard CBSE explanation:

• Orbits. The Sun's pull on a planet (or the Earth's pull on the Moon) is the centre-directed force that keeps it curving around instead of moving off in a straight line, as Newton's First Law would otherwise demand for an unforced body.
• Tides. Because $F$ depends on $r$, the Moon pulls the near side of Earth's oceans harder than the far side, producing two water bulges that sweep past a coastline roughly twice a day as the Earth spins.
• Atmosphere. The same pull, applied to air molecules instead of ocean water, is strong enough to hold the entire atmosphere against the Earth rather than letting it drift into space.
• Falling objects. Ordinary gravity — a dropped object accelerating toward the ground — is nothing but this law with one of the masses set to the whole Earth.

Every one of these phenomena is the same physics, just with different masses and distances plugged in — that is the entire "importance" of the law, and the reasoning pattern to reproduce in an answer is always the same three moves.

Identify the two masses. For orbits it is the Sun and a planet (or Earth and the Moon); for tides it is the Moon (and Sun) and Earth's ocean water; for the atmosphere it is the Earth and a gas molecule; for a falling object it is the Earth and that object.

State what the force does. A constant, centre-directed pull cannot fling an object away — by Newton's laws, a body moving sideways under a steady inward force is bent into a curved path rather than pushed outward. This is why the Moon does not fly off: it is continuously "falling" toward Earth, but its sideways velocity means the fall becomes a curve that keeps missing the Earth, producing a roughly circular orbit. Class 9 keeps this picture qualitative; the mathematics of circular motion is not required here.

Use the distance dependence. Because $F \propto 1/r^2$, force is not the same everywhere — it is stronger where $r$ is smaller. This single fact explains tides: the ocean water directly under the Moon (smallest $r$) is pulled hardest and bulges toward it, while the far-side water (largest $r$, and also less pulled than the solid Earth beneath it) is left behind as a second bulge. As the Earth rotates once every 24 hours, a coastline passes through both bulges, giving roughly two high tides a day. The same distance-dependence, applied to the Earth-and-atmosphere pair, explains why air close to the surface is held tightly while gravity's effect (though never zero) becomes negligible far out in space — never exactly zero, but weak enough at great distance that molecules bound tightly near the surface do not simply drift away.

A satellite is placed close to the Earth with the correct sideways speed for its altitude. Using the universal law of gravitation, explain why it keeps orbiting instead of falling straight down or flying off into space.

Step 1. Name the force. The Earth exerts $F = \dfrac{GM_{\text{Earth}}\,m_{\text{satellite}}}{r^2}$ on the satellite, directed toward the Earth's centre, exactly as the law predicts for any two masses.

Step 2. Consider the two extreme cases first, to see why neither happens. If the satellite had zero sideways velocity, this force alone would pull it straight down, and it would crash. If there were no gravitational force at all, Newton's First Law says it would move off in a straight line forever, never returning.

Step 3. Combine the two effects. The satellite does have sideways velocity, so at every instant gravity bends its straight-line path slightly toward the Earth. Given the right combination of speed and altitude, that continuous bending curves the path into a closed loop around the Earth rather than into it — the satellite is always "falling" but its forward motion means the ground curves away beneath it just as fast as it falls.

Answer: the satellite orbits because the Earth's gravitational pull continuously bends its otherwise straight-line motion into a curve that matches the curvature of the Earth, so it neither crashes nor escapes. (Class 9 does not require calculating the exact orbital speed — that is Class 11 content.)

• Thinking gravity "switches off" in space. The force never truly vanishes; it only weakens as $1/r^2$. Satellites and the Moon orbit because gravity keeps acting on them, not despite an absence of gravity.
• Believing only the Moon affects tides. The Sun contributes too, but the Moon dominates because it is far closer, even though the Sun has vastly more mass.
• Assuming an orbiting body feels "no force" since it never lands. It is under continuous gravitational force; it is falling continuously, but its sideways motion means it perpetually misses the Earth.
• Mixing this up with Class 11 material. Kepler's laws and the orbital-speed formula are not required here — Class 9 asks only for a qualitative explanation using $F = Gm_1m_2/r^2$.

Ask the tutor: Want to see how this same law is used later to calculate orbital speed and escape velocity, or explore more real-world examples of gravitation at work in the solar system?