Physics · Gravitation · NEET
The Sun is at one focus, NOT at the centre. An ellipse has two foci (F1 and F2). The Sun sits at one of them; the other focus is just an empty point in space with nothing at it. This single fact is the most common NEET trap, so remember: Sun = focus, never centre.
Gravity from the Sun is a central force (F = GMm/r^2 pointing toward the Sun). For such an inverse-square attraction, the bound path a planet traces is an ellipse. A circle is only the special case when the speed and distance are exactly balanced. Most real planets are slightly elliptical (for Earth, b/a is about 0.99986, so nearly circular but not exactly).
Perihelion is the point in the orbit closest to the Sun; aphelion is the point farthest from the Sun. Both lie on the major axis. The planet moves fastest at perihelion and slowest at aphelion (that speed change is actually Kepler's Second Law, but the two closest/farthest points come from the ellipse shape of the First Law).
Eccentricity e measures how stretched the ellipse is. e = 0 is a perfect circle; the closer e is to 1, the more flattened the ellipse. Planets have small e (Earth's e is about 0.017, so its orbit looks almost circular), while comets can have e close to 1 (very stretched).
No. An ellipse always has two foci because of its geometry, but only one focus holds the Sun. The second focus is an empty mathematical point. NEET sometimes shows a figure with two marked points to test whether you place the Sun correctly.
The kinetic energies of a planet in an elliptical orbit about the Sun at positions A, B and C are K_A, K_B and K_C. AC is the major axis and SB is perpendicular to AC at the Sun S (see figure). Then:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Every planet moves around the Sun in an elliptical orbit with the Sun at one focus of the ellipse.
Because it describes the shape (the orbit) that a planet follows, an ellipse, rather than the speed or the time period. The other two laws describe area sweep (Second Law) and time period (Third Law).
Yes. Any object bound by gravity to a much heavier body, such as an artificial satellite or the Moon around Earth, moves in an ellipse (or the special circular case) with the central body at one focus. This is why NEET links Kepler's laws to satellite motion.
No, but it is very close. Earth's orbit is a slightly flattened ellipse with eccentricity about 0.017, so it looks almost circular. It is still an ellipse, so Kepler's First Law holds exactly.
It fixes the geometry used in almost every Gravitation problem on planets, comets and satellites: ellipse shape, Sun at a focus, perihelion nearest and aphelion farthest. Getting this picture right lets you apply the Second and Third laws and energy formulas correctly.
