Because gravity depends on the mass of the objects, when large objects are involved then the force of gravity can be very strong. Gravity gets weaker as the objects get further apart. Because gravity depends on the mass of the objects, it plays a very
important part in keeping the planets in orbit around the Sun. Gravity is used to keepartificial satellitesin orbit. The satellite is positioned at a certain height above the Earth with a forward speed. This enables the satellite to maintain a constant height above the Earth in its gravitational field.
On which planet in the Solar System would gravity be strongest?
On which planet in the Solar System would the Sun’s gravity have the greatest effect?
Explain, using your ideas about gravity and orbits, why the planets in the Solar System remain in orbit around the Sun and do not start orbiting each other.
Before a satellite can be launched, its orbit needs to be carefully calculated. Why is it important to get these calculations correct?
… gravity … massive … orbit
195Newton’s Universal Law of Gravitation has been used to explain why it is that the planets do not follow precise orbits but wobble along their path. This is due to the attraction of other planets. It was the irregular motion of Uranus that led to the discovery of the planet
Neptune.
Black holes are formed when massive stars collapse under their own weight. The gravity is so strong that nothing, including light, can escape. In the photo below any surrounding gas and dust are spiralling into the black hole, attracted by the immense gravitational forces. The extreme conditions cause the gas and dust to heat up and glow.
Moon speed
gravity
Planet
A satellite placed in orbit is set in motion at the correct speed.
The force acting upon it is gravity, which is pulling it towards the planet It stays in motion because its direction is always changing
196
In August 2006, the International Astronomical Union decided that Pluto was not a planet.The evidence suggested that Pluto was not large enough to class as a planet. It is only 1/500th of the size of the Earth.
People’s ideas about our Solar System have changed over time. As technology has improved so has our understanding of what the Solar System is.
In ancient times, ideas about the stars were often the foundation of religious beliefs. Different civilisations had their own ideas to explain the apparent
movement of the stars across the sky. Stonehenge in Wiltshire is believed to be a very old solar calendar. We think this because of the way the stones and the Sun line up at certain times of the year.
The Mayans, in South America, developed a very advanced calendar based on observations of the planets, which enabled them to make their human sacrifices to the Gods at the right times of the year. The ancient Greek model suggested by Ptolemy in about AD150 put the Earth at the centre of the universe, with some of the planets and the Sun moving around it in circular orbits. This model became the standard model for 1500 years.
Throughout the Middle Ages most people believed that the Earth was flat and that you could fall off the edge of it.
A Polish priest called Nicolaus Copernicus (1473–1543) proposed that a
rotating Earth revolved around the Sun.This was the first time that a model with the Sun at its centre was proposed. Johannes Kepler (1571–1630) adjusted the model so that the orbits of the planets became egg- shaped or elliptical.
Galileo (1564–1642) invented the telescope and his
observations with it began to confirm Copernicus’ earlier theories.
Isaac Newton (1642–1727), devised a law of gravity which explained the motion of the planets across the sky. During
the late 1800s, the field of science desperately needed a new theory to revise the old Newtonian-based physics because it could not explain all the
observations that the more advanced telescopes were now making.
Albert Einstein (1879–1955) revolutionised all aspects of science and modern thought through a range of groundbreaking and unique theories which began to explain how gravity, light, time and space are
connected.
Stephen Hawking (1942– ) has made significant breakthroughs in our understanding of how the universe works by developing a quantum theory of gravity.
FIGURE 1:Stonehenge.
FIGURE 2:Ptolemy’s model
of the Universe.
ICT Activity
You are going to research some information and produce a timeline about the lives of these and other people who were important in the development of our ideas about the Solar System. You will need to find out when these people published their key ideas, and some more details about those ideas.
8
You recognise thatimproving our understanding of the Solar System and the universe will improve our knowledge about the world around us.
7
Your answers show anadvanced understanding of how our ideas about the Solar System have changed and the role of evidence in the development of explanations.
6
Your answers show agood understanding of how our ideas about the Solar System have developed and an appreciation of how the development of these ideas may have impacted on people’s lives both now and at the time.
5
Your answers show agood understanding of our planet and the Solar System. You understand about day, night and the seasons. You show a good grasp of some scientific terminology, and some appreciation of how these ideas impacted on people’s lives both now and at the time.
4
Your answers show abasic understanding of our ideas about the Solar System and a basic grasp of some scientific terminology.
197
Numeracy Activity
Use the Internet to find out the following facts about each planet:
● distance to the Sun (in millions of kilometres)
● average temperature on sunny side (°C)
Draw a line graph to show how the temperature varies with the distance from the Sun.
Which planet does not seem to fit the pattern? The asteroid belt is about 400 million km from the Sun. Use your graph to estimate the surface temperature of the asteroids.
Assess Yourself
1 On what evidence did ancient civilisations base
their ideas of the Solar System?
2 Where was the Sun in Ptolemy’s model?
3 What was the main change made by
Copernicus?
4 Why does Kepler’s work form the foundation of
the modern model of the Solar System?
5 Galileo invented the telescope. Why do you
think this made such a difference to our ideas about the planets?
6 Why do you think Einstein’s contribution was so
important?
7 Name the inner planets and give three
differences between them and the outer planets.
8 Why does the Moon stay in orbit around the
Earth?
9 What causes the seasons?
10 How has our knowledge of the Solar System got
Mass
Themassof an object simply means the amount of material the object
is made from. The more material the object is made from, the more mass it has. Things that have a big mass are harder to move and stop than objects with a small mass. So an empty bottle (with only air inside) is easier to move than a bottle filled with liquid. The full bottle contains more material so it has more mass than the empty bottle.
Your mass measures how much material your body has in it. If you eat too much your mass will increase. If you eat less, your mass will
decrease. In the metric system the unit of mass is thekilogram(kg).
What is mass? What units are used to measure it? Why are things with more mass harder to move?
Weight
Because the Earth has a large mass it produces a lot ofgravity.
Gravity always pulls things down: by ‘down’ we mean towards thecentreof the Earth. If this was not the case, people in Australia would fall off the Earth. The hard surface of the Earth stops you from sinking any further towards the centre of the planet. This is just as well.
Theforceof gravity acting on anything gives itweight.
Weight is a force that is measured innewtons.
Gravity on the Earth pulls every kilogram of mass with a force of 10 newtons.
So your weight on the Earth⫽ your mass
(in kilograms)⫻ 10
So mass and weight are related.
What is weight? What units are used to measure it?
What is the weight of a person of mass 65 kg?
In which direction does gravity act?