Page 20 - GALIET PHYSICS BLOSSOMS II+

- Now let’s consider a case where they will go wrong. Imagine a situation where we have 2 planetary orbits in the same plane around a star, one of which is roughly circular, and other of which is highly elliptical, so that the 2 orbits cross each other.
- Now suppose that at some time, they happen to both arrive at a crossing point at almost the same time (but not close enough to collide). What do you think will happen next?
- Show in an accurately drawn diagram what you think the planets will do, using your knowledge of Newton’s 2nd and 3rd laws, and also his law of gravitation. Explain the reasoning you use to get the resulting picture you show of the planetary motion. This example shows the big difference between Kepler’s laws, which were really just a set of rules, and Newton’s laws, which provided a universal mechanical explanation of motion.
What Will the Planets Do?
By Newton’s Law of Gravity the Sun and the planets attract each other gravitationally ensuring that planetary orbits remain stable. This gravitational force pulls each planet towards the Sun, deflecting its tendency to move on a straight path by turning it into a curved orbital path.
Figure 1.11
In other words, when the Sun’s inward gravity pulls on a planet, it competes with the planet’s tendency to go on moving in a straight line according to Newton’s first Law of inertia (the tendency of an object to keep moving in the same direction, at the same speed unless a force acts on it, in this case, gravity). Both effects cause the planet to orbit smoothly around the sun along an intermediate path. This tension between the Sun’s gravity and the planet’s inertia stabilizes planetary orbits.
11 Drawing from Fig. 2.23 Chaisson, McMillan. Astronomy Today. 50. •20•