Ch 6Gravitation — Class 11 Physics, concept-first.

For most of ancient and medieval history, people believed the Earth stood still at the centre of the universe while the Sun, Moon and planets circled around it.

Kepler's three laws, extracted purely from Tycho Brahe's observational data (with no underlying force law behind them yet), completely describe planetary orbits.

Kepler's laws described planetary motion perfectly but gave no reason why planets move that way. Newton supplied the reason: a single, universal force law.

The size of is what decides how strong gravity actually is between any two given masses -- and it is a very, very small number, which is exactly why gravity between two everyday objects (say, two peop…

Two masses like the Sun and the Earth are never in physical contact with each other, and yet the Earth clearly experiences a huge, continuous pull toward the Sun.

Definition. The gravitational field intensity (or simply the gravitational field) at a point that is a distance from a mass is defined as the gravitational force experienced by a unit mass placed at t…

The superposition principle. When different masses are all present, the total gravitational field they produce together at any point is simply the vector sum of the individual fields each one would pr…

Since the physical meaning of potential energy was already introduced for springs and for gravity-near-the-surface in an earlier unit, and since the gravitational force is a conservative force, we can…

The formula , used constantly for objects near the Earth's surface, is not an independent rule -- it is simply the general result approximated for the special case where the height above the surface i…

Definition. The gravitational potential at a distance from a mass is defined as the amount of work required to bring a unit mass from that point out to infinity -- equivalently, it is the gravitationa…

Every falling object near the Earth's surface accelerates downward at the same rate, regardless of its own mass -- this is a direct consequence of combining Newton's law of gravitation with his second…

The value is only exact right at the Earth's surface near the equator. Moving up, down, or sideways to a different latitude all change the measured value slightly.

Earth's atmosphere is mostly nitrogen and oxygen, even though hydrogen and helium are by far the most abundant elements in the universe.

A satellite stays in orbit because Earth's own gravity supplies exactly the centripetal force needed to keep it curving around the planet rather than flying off in a straight line.

A satellite's total mechanical energy is the sum of its kinetic energy and its gravitational potential energy, and this sum always comes out negative -- which is the mathematical statement that the sa…

Satellites orbiting at different heights have different orbital periods (from Kepler's third law, Section 6.4.1) -- so by choosing the right height, a satellite's period can be made to match any value…

Your everyday sense of "weight" and the actual gravitational force pulling on you are subtly different things, and understanding that difference explains the jerk you feel starting or stopping in a li…

Astronomy is one of the oldest sciences in human history, and for most of that history it was an inseparable part of physics -- indeed, Kepler's laws and Newton's law of gravitation were themselves fo…

When the planets are tracked against the background stars over several months, most of them are seen to drift steadily eastward -- but every so often, a planet like Mars appears to slow down, reverse…

Kepler leaned heavily on Tycho Brahe's naked-eye positional data to formulate his third law -- and astronomers of the same era used pure geometry and trigonometry to measure the actual distances from…

Around 225 BC, the Greek scholar-librarian Eratosthenes, working in Alexandria, measured the Earth's radius to a small error compared with modern measurements -- using only two vertical sticks, their…

Four short case studies showing how simple photography, geometry, and careful nighttime observation reveal genuine facts about the Earth and the Moon.

Until the end of the 19th century, astronomy relied on naked-eye or telescopic observation alone. The discovery of the wider electromagnetic spectrum then opened up entirely new ways to observe the un…

- The motion of planets is fully described by Kepler's three laws: (1) every planet orbits the Sun in an ellipse with the Sun at one focus; (2) the radial vector from the Sun to a planet sweeps out eq…

The unit's ideas branch out from one starting point and converge on satellites:

This closing exercise set tests every idea developed in the unit, across five question formats:

Sample papers and previous-year board questions for this subject.