Space and gravity, Year 9

200 N

HintEvery kilogram of her is pulled down with the force given for that planet.

The whyWeight = mass × g, so 50 × 4. On Earth, where g is 10 N/kg, the same astronaut weighs 500 N. She is the same person with the same mass; only the pull on each kilogram has changed.

80 kg

HintAsk whether the journey has added or removed any of the matter she is made of.

The whyMass is the amount of matter in an object, and travelling does not change it. What changes on the Moon is her weight, the force of gravity on that mass, which is about a sixth of its value on Earth.

500 N

HintFind the one thing about the probe that stays fixed wherever it goes, then use the new planet's pull.

The whyFirst find the mass: 200 ÷ 10 = 20 kg. That mass is the same at Jupiter, so the weight there is 20 × 25. Going by way of the mass is the safe route between any two worlds.

The Moon pulls her down with a smaller force

HintHer legs push just as hard as at home, so ask what is working against them.

The whyThe Moon's gravitational field strength is about a sixth of the Earth's, so her weight is about a sixth of what it was. The same push from her legs is opposed by a much weaker downward force, and she rises further before it brings her back.

It is sunlight reflected off the planet

HintVenus makes none itself, so think about what is shining on it.

The whyPlanets are not hot enough to glow visibly. We see them the same way we see the Moon: light from our star falls on them and some bounces back towards us. That is one way to tell a planet from a star, however alike they look to the eye.

A star

HintOur solar system has exactly one, at its centre.

The whyThe Sun is one of these seen from close by, and every point of light in a constellation is another seen from very far away. Many of them are bigger and brighter than the Sun; distance alone makes them look like pinpricks.

Sun

HintYou can see it without waiting for night.

The whyIt is easy to forget that the bright disc in the daytime sky belongs in the same family as the points of light at night. The next nearest is so much further away that its light takes over four years to arrive.

galaxy; universe

HintEach step up contains everything that came before it.

The whyThe Earth is one planet among several around the Sun. The Sun is one star among billions in the Milky Way. The Milky Way is one among billions of similar collections. Each step is a vast jump in size.

A galaxy

HintAndromeda is a famous one, and we live inside another.

The whyStars are not scattered evenly through space. They are gathered into these great systems, with almost empty space in between. Seen from far outside, a whole one looks like a single faint smudge of light.

The Milky Way

HintOn a very dark night it shows as a pale, cloudy band right across the sky.

The whyIt is a flat spiral, and the Sun sits out in one of its arms, roughly halfway from the centre to the edge. The band we see at night is our own galaxy viewed edge-on from the inside.

Billions

HintFar more than there are people on Earth.

The whyEstimates for our own galaxy run to at least a hundred thousand million stars. Only a few thousand are bright enough and near enough to see without a telescope, so the night sky shows a tiny sample.

50

HintThe news can only reach us by travelling here as light.

The whyA light year is the distance light covers in one year, so light from something 50 of them away takes 50 years to make the trip. Until that light arrives, the star would look exactly as it always has.

Its light set off long ago

HintThink about how old the picture is by the time it reaches your telescope.

The whyWe never see a distant object as it is now, only as it was when the light we are receiving left it. For a galaxy millions of light years away, that light began its journey millions of years ago.

In kilometres the numbers would be unmanageably huge

HintTry writing the distance to the next star using a unit made for road journeys.

The whyOne light year is about 9.5 million million kilometres, and even the nearest star beyond the Sun is more than four of them away. A unit sized for the job keeps the numbers small enough to read and compare.

4

HintMore than the 8 minutes sunlight takes, but fewer than ten years.

The whyThat star is a little over four light years away. Light from the Sun arrives in about eight minutes, which shows how much emptier the space between stars is than the space inside our solar system.