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Gravity and Orbits in the Solar System — Answers

Answer sheet with model answers and success criteria for the Year 8 the solar system & space worksheet.

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Success TutoringWorksheetsAnswer sheetYear 8 · Science · The Solar System & Space

Gravity and Orbits in the Solar System — Answers

Part A: What is gravity?

Read the passage, then answer the questions.

Gravity is a non-contact force that pulls objects with mass towards each other. The more mass the objects have, and the closer they are, the stronger the pull.

The Sun has an enormous mass, so its gravity is very strong. Each planet is moving forward very fast, and the Sun's gravity pulls it inwards. Together these produce a curved path called an orbit. In the same way, Earth's gravity keeps the Moon in orbit around Earth.

1.Which statement about gravity is correct?
  1. AOnly Earth has gravity.
  2. BEvery object with mass pulls on every other object with mass.
  3. CGravity only works when objects touch.
  4. DGravity pushes objects apart.

Answer: Every object with mass pulls on every other object with mass.

Gravity is a non-contact force of attraction between all masses. It is only noticeable when at least one object has a huge mass, like a planet or star.

2.The gravitational pull between two objects gets stronger when …
  1. Athe objects move further apart
  2. Bthe objects have more mass or are closer together
  3. Cthe objects are colder
  4. Dthe objects spin faster

Answer: the objects have more mass or are closer together

More mass and less distance both increase the force of gravity.

3.Why do the planets orbit the Sun instead of flying off into space?
  1. AThe Sun's magnetism holds them.
  2. BThe Sun's gravity pulls them towards it while they move forward, so they travel in a curved path.
  3. CThey are tied by solar wind.
  4. DSpace is too thick for them to escape.

Answer: The Sun's gravity pulls them towards it while they move forward, so they travel in a curved path.

Each planet is moving forward very fast. The Sun's gravity constantly pulls it inwards, bending its path into an orbit.

4.The Sun contains more than 99% of the mass of the solar system, so its (answer space) controls the orbits of all the planets.

Answer: gravity

5.Without the Sun's gravity, Earth would travel off into space in a straight line.

Answer: True

With no force pulling it inwards, Earth would keep moving in a straight line at a steady speed.

6.The Moon orbits Earth because of Earth's gravity.

Answer: True

Earth's gravity pulls on the Moon, keeping it in orbit, and the Moon's gravity in turn causes Earth's tides.

Part B: Mass and weight

Show your working.

Mass is the amount of matter in an object, measured in kilograms (kg). It is the same everywhere. Weight is the force of gravity on an object, measured in newtons (N). It changes depending on where you are.

weight (N) = mass (kg) × gravitational field strength (N/kg). On Earth the field strength is about 10 N/kg (more precisely 9.8 N/kg).

7.A student has a mass of 50 kg. What is their weight on Earth (10 N/kg)?

Answer: 500 N

  1. weight = 50 × 10 = 500 N
8.What would the same student weigh on the Moon (1.6 N/kg)?

Answer: 80 N

  1. 50 × 1.6 = 80 N
9.What would they weigh on Mars (3.7 N/kg) and on Jupiter (25 N/kg)?

Answer: Mars 185 N; Jupiter 1250 N

  1. 50 × 3.7 = 185 N
  2. 50 × 25 = 1250 N
10.What is the student's mass on the Moon?
  1. A8 kg
  2. B50 kg
  3. C80 kg
  4. D0 kg

Answer: 50 kg

Mass is the amount of matter and does not change from place to place. Only weight changes.

11.Astronauts floating in a space station orbiting Earth feel weightless because there is no gravity there.

Answer: False

Gravity at that height is still about 90% as strong as on the ground. Astronauts float because they and the station are falling around Earth together.

Part C: Patterns in orbits

Planet data (1 astronomical unit = the average distance from Earth to the Sun, about 150 million km)

Mercury: 0.39 astronomical units from the Sun, one orbit takes 88 Earth days

Earth: 1.0 astronomical unit, 365 Earth days

Mars: 1.5 astronomical units, 687 Earth days

Jupiter: 5.2 astronomical units, about 12 Earth years

Neptune: 30 astronomical units, about 165 Earth years

12.Describe the pattern between a planet's distance from the Sun and the time it takes to orbit once.

Answer: The further a planet is from the Sun, the longer it takes to complete one orbit (the longer its year).

13.About how many Earth years does Mars take to orbit the Sun once? Round to one decimal place.

Answer: About 1.9 Earth years

  1. 687 ÷ 365 ≈ 1.88
  2. ≈ 1.9 Earth years
14.Use ideas about gravity and distance to suggest two reasons why Neptune takes so much longer than Earth to orbit the Sun.

Model answer: First, Neptune is about 30 times further from the Sun than Earth, so its orbit is a much larger circle and it has a far greater distance to travel. Second, because it is so far away, the Sun's gravity pulling on it is much weaker, so Neptune moves more slowly along its orbit. A longer path travelled at a slower speed means a much longer year — about 165 Earth years.

Success criteria:

  • Longer orbital path because of greater distance
  • Weaker gravity at greater distance
  • Links weaker gravity to slower orbital speed
  • Uses the data to support the answer
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