Motion in the Universe Notes

Edexcel IGCSE Physics: Revision notes

Key facts

The universe contains billions of galaxies; a galaxy holds billions of stars; our Sun is in the Milky Way.
Gravitational field strength $g$ depends on the mass of the body and the distance from its centre.
Gravity provides the force that keeps moons, planets, satellites and comets in orbit.
Planet and moon orbits are nearly circular; comet orbits are highly elliptical.
Orbital speed $v = \dfrac{2\pi r}{T}$ .

The universe and galaxies

The universe is billions of galaxies. Each galaxy is billions of stars held together by gravity, and our Sun is in the Milky Way.

A galaxy is a huge collection of billions of stars held together by gravity.

Our Sun and Solar System are in the Milky Way galaxy, which is one of billions of galaxies in the universe.

1
Universe
billions of galaxies
2
Galaxy
billions of stars, held together by gravity
3
Milky Way
our galaxy, containing the Sun and Solar System

Scale of the universe

What holds the stars of a galaxy together?

Gravitational field strength

Gravitational field strength is the force of gravity per unit mass. It is bigger for a more massive body and smaller at greater distance from its centre.

Gravitational field strength $g$ differs from place to place. The Moon has much less mass than Earth, so its $g$ is smaller.

Weight $W = mg$ changes with $g$ , but mass stays the same everywhere.

Gravitational field strength at the surface.

Worked example

Find the weight of a 60 kg astronaut on the Moon ( $g = 1.6$ N/kg).

1
$W = mg$ .
2
$W = 60 \times 1.6$ .

Why is g smaller on the Moon?

What keeps things in orbit

Gravity pulls an orbiting body towards the central mass. Without it, the body would travel in a straight line.

Gravitational force provides the centripetal force that keeps an object on a curved path. It makes moons orbit planets, planets orbit the Sun, satellites orbit Earth and comets orbit the Sun.

Its velocity is along the tangent, while the force points to the centre.

Gravity pulls a planet towards the Sun; its velocity is along the tangent.

Moons orbit

Planets

Planets orbit

The Sun

Satellites orbit

Earth

Comets orbit

The Sun

If gravity suddenly switched off, a planet would

Shapes of orbits

Planets and moons follow nearly circular orbits. Comets follow highly elliptical orbits, so their distance and speed change a lot.

A comet comes very close to the Sun at one point and travels far away at another.

Its distance from the Sun, and so its orbital speed, varies hugely in one orbit. A planet's speed is roughly steady.

A comet's orbit: a very elongated ellipse (not to scale).

Planets

Nearly circular
Roughly constant distance

Moons

Nearly circular
Orbit their planet

Comets

Highly elliptical
Very close, then very far

Which body has a highly elliptical orbit?

Orbital speed

In a circular orbit, speed is the circumference divided by the time for one orbit.

In one orbit, a body travels the circumference, $2\pi r$ , in one time period $T$ . Average speed is distance divided by time.

$r$ is the orbital radius and $v$ is the orbital speed.

A circular orbit: in one time period T the body travels the circumference 2πr, always moving along the tangent.

Orbital speed $v = \dfrac{2\pi r}{T}$

Worked example

A satellite orbits at a radius of $7 \times 10^6$ m with a time period of 6000 s. Find its orbital speed.

1
$v = \dfrac{2\pi r}{T}$ .
2
$v = \dfrac{2\pi \times 7 \times 10^6}{6000}$ .

The orbital radius doubles but the period stays the same. The speed

Try an exam question

A satellite is in a circular orbit around the Earth. The orbital radius is $7.0 \times 10^6$ m and the time period is 6000 s. State what provides the force that keeps the satellite in orbit and calculate its orbital speed.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.