Gravity & Friction — Drills · Grade 8 Science

Chapter: Forces

What these drills covers

By the end of Gravity & Friction you should be able to:

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

Vocabulary used: gravity, weight, mass, friction, air resistance, model, evidence, system boundary, assumption.

Problem 1

Friction always acts
  1. in the direction of motion
  2. opposite to the relative motion or attempted motion
  3. downward
  4. upward
Answer and explanation

opposite to the relative motion or attempted motion

Friction opposes sliding between surfaces, so it points against the motion or the attempted motion.

Problem 2

Gravitational attraction between two objects increases when
  1. their masses increase
  2. they move further apart
  3. their masses decrease
  4. they stop moving
Answer and explanation

their masses increase

Greater mass means greater attraction; greater distance weakens it.

Problem 3

An object weighs 98 N on Earth where g is about 9.8 N/kg. What is its mass in kilograms?
Answer and explanation

10 kg

mass = weight / g = 98 / 9.8 = 10 kg.

Problem 4

A 6.0 kg object sits on Earth where g is 9.8 N/kg. What is its weight in newtons?
Answer and explanation

58.8 N

weight = mg = 6.0 × 9.8 = 58.8 N.

Problem 5

An astronaut's mass on the Moon compared with on Earth is
  1. smaller
  2. the same
  3. larger
  4. zero
Answer and explanation

the same

Mass is the amount of matter and does not change with location; weight changes because gravitational field strength differs.

Problem 6

Which change would reduce friction between two sliding surfaces?
  1. Pressing them together harder
  2. Adding a lubricant
  3. Roughening them
  4. Increasing the contact force
Answer and explanation

Adding a lubricant

A lubricant separates the surfaces so less direct contact occurs.

Problem 7

Explain the difference between mass and weight, and why the distinction matters when comparing Earth and the Moon.
Answer and explanation

Mass measures how much matter an object contains and is the same everywhere; weight is the force of gravity on that mass and depends on the local gravitational field strength. On the Moon the field is about a sixth of Earth's, so an object's weight is about a sixth while its mass is unchanged. Confusing the two leads to the mistake that objects 'lose mass' in space.

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

A sled is easier to keep moving than to start moving. Explain in terms of friction.
Answer and explanation

The friction that must be overcome to start it moving — static friction — is generally larger than the friction acting once it slides. At rest the surfaces settle into closer contact, so a larger force is needed to break them free; once sliding, the contact is less complete and the opposing force is smaller, so a smaller push maintains the motion.

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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