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Balls and Potatoes: Why Some Moons Are Round worksheet preview

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Balls and Potatoes: Why Some Moons Are Round

Grade 8 gravity practice: a podcast club uses probe data on moons and asteroids to argue that gravity attracts and depends on mass.

Grade 8ScienceInteractive · Printable

Aligned toMS-PS2-4

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What’s included

  • Single worksheet
  • Auto-graded online
  • 1 NGSS standard
  • Printable answer key
  • 10 questions
  • Classroom-ready

Details

Format
Interactive · Printable
Grade
Grade 8
Subject
Science
Question types
Fill in the blank, Multiple choice, Matching, Circle the answer, Sorting, Short answer

About this worksheet

What students practice

  • Students use data on moons and asteroids to show that bodies with more mass pull harder.
  • Students argue from evidence that gravity attracts and depends on the masses of both objects.

How to use

Read the episode notes and the table together first, then have students answer on their own and share their arguments for the last question.

Curriculum

NGSS standard covered

MS-PS2-4

Argue from evidence that gravity is attractive and depends on the masses of the objects.

Preview

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Name Date

Balls and Potatoes: Why Some Moons Are Round

Use space probe data to argue about gravity and mass.

The probe data

Read the podcast notes and the table. Use them to answer the questions.

Episode Notes: Balls and Potatoes in Space

1The science podcast club at Corbin Valley Middle School records a short episode every two weeks to answer a question from a listener. This month the question came from a sixth grader named Ilse: "In photos from space probes, big moons look like smooth balls, but small moons and asteroids look like lumpy potatoes. Why is that?" Eighth graders Tamsin Okeke and Rasmus Aguilar took on the question with help from their science advisor, Ms. Beatrix Nwosu.

2Tamsin started with data that space probes have gathered over the past fifty years. For six bodies in the solar system, she listed the width, the mass and the pull of gravity on a 1 kg rock resting on the surface. She also noted what each body orbits and how it looks in probe photos. Ms. Nwosu pointed out that rock and ice seem hard and stiff, but under a strong, steady squeeze that lasts millions of years they can slowly sag and flow, a little like very thick putty.

3Rasmus looked for clues about loose material on small bodies. Close-up photos of the asteroid Itokawa, which is only about 0.5 km long, show that loose gravel has collected in its low, smooth areas, while big boulders cover its higher parts. A probe that visited the asteroid Bennu, also about 0.5 km wide, found that it is a loose pile of boulders and gravel with empty gaps between the pieces. Scientists think that nothing glues most of these pieces to each other.

4Ms. Nwosu added one more fact. On any one body, a rock with more mass is pulled harder. On Ceres, for example, the pull on a 1 kg rock is about 0.28 N, and the pull on a 2 kg rock is about 0.56 N.

5Before recording, the two hosts agreed on a rule for their episode: every part of their answer to Ilse had to rest on evidence from the data or the probe photos, not on a guess. Tamsin planned to open the episode with the table, and Rasmus wanted to close it with the asteroid photos. They also hoped to answer a second question that listeners often send in: does gravity really act on something as small as an asteroid?

Tamsin's Table: Six Bodies Seen by Space Probes
BodyOrbitsWidth (km)Mass (kg)Surface pull on 1 kg (N)Shape in photos
Earth's MoonEarth3,4747.3 × 10²²1.6Round
CeresThe Sun9409.4 × 10²⁰0.28Round
EnceladusSaturn5041.1 × 10²⁰0.11Round
MimasSaturn3963.8 × 10¹⁹0.064Round
HyperionSaturn2705.6 × 10¹⁸0.02Lumpy
PhobosMars221.1 × 10¹⁶0.0057Lumpy
1.
Use the table. Suppose a student with a mass of 50 kg stood on the surface of Ceres.

Gravity on Ceres would pull on the student with a force of about   N.

2.
Which pattern does the table show?
  • A. The more mass a body has, the harder it pulls on a 1 kg rock at the surface.
  • B. The wider a body is, the weaker its pull on a 1 kg rock at the surface.
  • C. The pull on a 1 kg rock is about 0.3 N on each body, since the rock is the same.
  • D. The lumpy bodies pull harder on a 1 kg rock than the round bodies do.
3.
Use the table. Compare the surface pull on a 1 kg rock on Earth's Moon and on Mimas.

The pull on Earth's Moon is   times the pull on Mimas.

4.
Use the table and paragraph 2. Which explanation of why the Moon, Ceres, Enceladus and Mimas are round, while Hyperion and Phobos are lumpy, fits the evidence best?
  • A. The lumpy bodies have the strongest pull at the surface, so their rock is piled up into high peaks.
  • B. The round bodies orbit planets, and a planet's pull slowly shapes each of its moons into a ball.
  • C. The lumpy bodies push their loose rock outward, so the rock stacks up into bumps on the surface.
  • D. The round bodies have much more mass, so their own pull drew their rock and ice in toward the center.
5.
Suppose a probe discovers each body below. Use the pattern in the table to sort each one by the shape it most likely has.
An icy moon 1,100 km wide with a mass of 1.2 × 10²¹ kgA rocky moon 30 km wide with a mass of 2 × 10¹⁶ kgAn icy moon 750 km wide with a mass of 3 × 10²⁰ kgAn asteroid 2 km wide with a mass of 1 × 10¹³ kgAn icy moon 150 km wide with a mass of 1 × 10¹⁸ kg
Most likely roundMost likely lumpy

Gravity on small bodies

Use the podcast notes and the table to answer the questions.

6.
Circle the two observations that are evidence that even a very small body pulls loose material toward itself.

Loose gravel on Itokawa has collected in its low, smooth areas.Itokawa is only about 0.5 km long.Bennu's boulders and gravel stay together, though nothing glues most of them.Hyperion orbits the planet Saturn.Space probes have gathered data for about fifty years.

7.
Use paragraph 4. On Ceres, the pull on a 1 kg rock is about 0.28 N, and the pull on a 2 kg rock is about 0.56 N. What does this show?
  • A. Ceres pulls on the 2 kg rock, but the 1 kg rock is too small to be pulled.
  • B. Ceres pulls on both rocks with the same force, since they rest on the same body.
  • C. The pull depends on the rock's mass as well as on the mass of Ceres.
  • D. The pull is twice as strong on the 1 kg rock, since a lighter rock is easier to pull.
8.
Suppose a newly found icy moon is about as wide as Mimas, but it is full of holes and has only about one tenth of Mimas' mass. Based on the table, which prediction is best?
  • A. Its surface pull would match Mimas', since the two moons are about the same width.
  • B. Its surface pull would be weaker than Mimas', and it would more likely be lumpy.
  • C. Its surface pull would be stronger than Mimas', since gravity passes through holes easily.
  • D. It would be rounder than Mimas, since a moon with less mass is easier to squeeze.
9.
Match each observation to the idea about gravity it supports best.
  1. Gravel collects in the low areas of Itokawa.
  2. Ceres pulls a 2 kg rock twice as hard as a 1 kg rock.
  3. The same 1 kg rock is pulled far harder on Earth's Moon than on Phobos.
  1. A. Gravity pulls loose material toward a body.
  2. B. A moon or asteroid with more mass pulls harder.
  3. C. The pull depends on the mass of the rock being pulled.
10.
Ilse writes back: "My brother says small asteroids are lumpy because gravity works on big things like planets and large moons, but not on small things." Do you agree with her brother? Write a claim, give evidence from the table and paragraphs 3 and 4, and explain your reasoning about how gravity depends on mass.

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