Gravity in the solar systemIB MYP Physics: Revision notes

Every object attracts every other object with a gravitational force. The force is:

• larger when the masses are larger
• smaller when the objects are further apart

The force falls off quickly with distance. If the distance is doubled, the force becomes one quarter of its original size. This is the inverse-square idea. The Sun's pull on Neptune is therefore far weaker than its pull on Earth.

An orbit is the curved path of one object around another. Gravity holds:

• planets in orbit around the Sun
• moons in orbit around planets
• satellites in orbit around the Earth

Gravity acts towards the centre of the orbit, at right angles to the object's direction of motion. It does not change the speed, but it keeps changing the direction, so the object follows a curve instead of a straight line.

The orbital period is the time for one complete orbit. For objects orbiting the same body:

• the closer the orbit, the faster the object moves and the shorter the period
• the further away, the slower it moves and the longer the period, because the gravitational pull is weaker and the path is longer

Mercury orbits the Sun in 88 days, Earth in 365 days and Neptune in about 165 years. Satellites in low orbit, such as the ISS, take about 90 minutes.

An object is in free fall when gravity is the only force acting on it. It accelerates towards the centre of the planet.

An orbiting object is in free fall. It falls towards the Earth, but because it moves sideways at high speed, the Earth's surface curves away beneath it, so it keeps falling around the Earth without hitting it.

Astronauts on the International Space Station seem to float, but gravity there is still about 90% of its value at the surface. They are not far enough away to escape it.

The astronauts and the station are in free fall together, so the floor does not push on the astronauts. They feel weightless, but their mass is unchanged and gravity still acts on them.