Gravity, Orbits & Tides — Drills · Grade 9 Science

Chapter: Earth, Moon & Solar System

What these drills covers

By the end of Gravity, Orbits & Tides you should be able to:

8 questions, multiple choice, numeric and written answer, each with a worked explanation.

Vocabulary used: gravity, orbit, tide, spring tide & neap tide.

Problem 1

A planet stays in orbit because
  1. there is no gravity in space
  2. gravity continuously changes its direction of motion
  3. it is pushed by solar wind
  4. it has no mass
Answer and explanation

gravity continuously changes its direction of motion

Gravity provides the centripetal force that bends a straight path into an orbit.

Problem 2

Gravitational force between two bodies increases if
  1. masses increase or distance decreases
  2. masses decrease
  3. distance increases
  4. they stop moving
Answer and explanation

masses increase or distance decreases

It grows with mass and falls rapidly with separation.

Problem 3

High tides occur roughly
  1. once a day
  2. twice a day in most places
  3. once a week
  4. once a month
Answer and explanation

twice a day in most places

Bulges on both the near and far sides of Earth mean most coasts pass through two each day.

Problem 4

Spring tides, with the largest range, occur when the Sun and Moon are
  1. at right angles
  2. aligned with Earth
  3. both below the horizon
  4. closest to Earth only
Answer and explanation

aligned with Earth

At new and full moon their tidal effects add together.

Problem 5

Astronauts float in orbit because
  1. there is no gravity there
  2. they and the station are both falling freely around Earth
  3. they are weightless objects
  4. the station blocks gravity
Answer and explanation

they and the station are both falling freely around Earth

Gravity is still strong in low orbit; continuous free fall produces the floating.

Problem 6

The Moon orbits Earth in about how many days, to the nearest day?
Answer and explanation

27 days

About 27.3 days relative to the stars, slightly less than the 29.5-day phase cycle.

Problem 7

Explain why the Moon produces larger tides than the Sun despite the Sun's far greater mass.
Answer and explanation

Tides are produced by the difference in gravitational pull across Earth's diameter rather than by the total pull. That difference falls off very steeply with distance, so the Moon's closeness outweighs the Sun's much greater mass. The Sun's contribution is real but roughly half the Moon's, which is why the two combine into larger spring tides when aligned.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

Problem 8

Explain why a satellite needs a particular speed to stay in a given orbit.
Answer and explanation

Orbiting means falling toward Earth while moving sideways fast enough that the surface curves away at the same rate. Too slow and the satellite's path intersects the ground; too fast and it climbs to a higher orbit or escapes. For each orbital radius there is one speed at which the fall and the curvature match, which is why orbits are changed by altering speed.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

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