Know that the force that keeps an object in orbit around the Sun is the gravitational attraction of the Sun

6.1.2 The Solar System

Objective

Know that the force that keeps an object in orbit around the Sun is the gravitational attraction of the Sun. 🌞

What keeps the planets moving? 🤔

The Sun’s gravity pulls every planet toward it. The pull is strong enough to keep the planets from flying straight out into space, but not so strong that they crash into the Sun. The result is a delicate balance: the planet’s forward motion keeps it from falling straight down, while the Sun’s pull bends that motion into a smooth orbit. This is similar to how a skateboarder spins around a pole while staying on the ground. 🚀

The mathematical description is:


\$F = G \frac{M{\text{Sun}}\,m{\text{planet}}}{r^2}\$

where:

  • \$G\$ = gravitational constant

  • \$M_{\text{Sun}}\$ = mass of the Sun

  • \$m_{\text{planet}}\$ = mass of the planet

  • \$r\$ = distance between the Sun and the planet

Analogy: The Sun as a Giant Magnet

Imagine the Sun is a huge magnet. The planets are like iron filings that are attracted to the magnet. The filings move around the magnet because they are pulled in one direction (toward the magnet) but also have their own forward motion. The result is a circular path around the magnet. 🌍

Key Facts About the Solar System

PlanetAverage Distance from Sun (million km)Orbital Period (years)
Mercury57.90.24
Venus108.20.62
Earth149.61.00
Mars227.91.88

Exam Tips 📚

  • Remember the formula for gravitational force and the symbols used.

  • Use the analogy of a magnet or a skateboarder to explain why planets stay in orbit.

  • Show the relationship between distance and orbital period using Kepler’s third law if asked.

  • When drawing diagrams, label the Sun, the planet, the radius vector, and the direction of motion.

  • Practice converting units (e.g., km to m) when calculating forces.