Law of orbits : All planets move in elliptical orbits with the Sun situated at one of the foci of the ellipse (Fig.1). This law was a deviation from the Copernican model which allowed only circular orbits. The ellipse, of which the circle is a special case, is a closed curve which can be drawn very simply as follows.
Select two points F1 and F2. Take a length of a string and fix its ends at F1 and F2 by pins. With the tip of a pencil stretch the string taut and then draw a curve by moving the pencil keeping the string taut throughout.(2).
The closed curve you get is called an ellipse. Clearly for any point T on the ellipse, the sum of the distances from F1 and F2 is a constant. F1, F2 are called the focii. Join the points F1 and F2 and extend the line to intersect the ellipse at points P and A as shown in Fig. (2). The midpoint of the line PA is the centre of the ellipse O and the length PO = AO is called the semimajor axis of the ellipse. For a circle, the two focii merge into one and the semi-major axis becomes the radius of the circle.
Law of areas : The line that joins any planet to the sun sweeps equal areas in equal intervals of time (Fig). This law comes from the observations that planets appear to move slower when they are farther from the sun than when they are nearer.
Law of periods : The square of the time period of revolution of a planet is proportional to the cube of the semi-major axis of the ellipse traced out by the planet.(185)Related posts :
Moment of inertia Torque Centre of mass
Friction introduction Rolling Friction Newton's First law of motion Newton's second law of motion Newton's third law of motion
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