AQA A-Level Physics: Gravitational Field Lines and Gravitational Field Strength — mark scheme explained

The question

Mark scheme, decoded

How the examiner actually awards the marks on this topic.

Gradora's own decode of the marking approach — not the exam board's published mark scheme.

Model answer

A full-mark response to the question above, worked through step by step.

⏱Timing: Allocate about 5-7 minutes for each question involving calculations or derivations, and 3-4 minutes for explanation questions.

1. Identify the given values: M = 7 × 10 23 kg, r = 4,000 km = 4,000,000 m, and G = 6.674 × 10 -11 N·m 2 /kg 2 .1 mark
2. Use the formula for gravitational field strength in a radial field: g = GM r 21 mark
3. Substitute the values into the formula: g = (6.674 × 10 -11 ) (7 × 10 23 ) (4,000,000) 21 mark
4. Calculate the numerator: (6.674 × 10 -11 ) (7 × 10 23 ) = 4.6718 × 10 13 N·m 2 /kg1 mark
5. Calculate the denominator: (4,000,000) 2 = 1.6 × 10 13 m 20 marks
6. Divide the numerator by the denominator: g ≈ 4.6718 × 10 13 1.6 × 10 130 marks
7. Simplify the result: g ≈ 2.92 m/s 22 marks

Final answer: 2.92 m/s 2

✓Work through every step correctly and you earn all 6 marks.

Common mistakes

• Confusing the units of gravitational field strength

  Why it happens: Students often mix up the units of gravitational field strength, using N instead of N/kg or m/s 2 .

  Fix: Always remember that the unit of gravitational field strength is newtons per kilogram (N/kg) or meters per second squared (m/s 2 ).

• Using the wrong formula for gravitational field strength

  Why it happens: Students sometimes use the formula for gravitational force instead of the formula for gravitational field strength.

  Fix: Ensure you are using the correct formula: g = GM r 2 . The gravitational force formula is F = Gm 1 m 2 r 2 .

• Forgetting to convert distances from kilometers to meters

  Why it happens: Students often forget to convert the distance r from kilometers to meters when using the formula for gravitational field strength.

  Fix: Always ensure that all distances are in meters before substituting them into the formula. For example, 10,000 km = 10,000,000 m.

• Misinterpreting the direction of gravitational field lines

  Why it happens: Students sometimes think that gravitational field lines point away from a mass rather than towards it.

  Fix: Gravitational field lines always point towards the mass, indicating the attractive nature of gravity. This is different from electric field lines, which can point away from positive charges.

• Using the wrong value for the gravitational constant

  Why it happens: Students sometimes use an incorrect value for the gravitational constant G or forget to include it in their calculations.

  Fix: Always use the correct value of the gravitational constant: G = 6.674 × 10 -11 N·m 2 /kg 2 . Double-check that you have included it in your calculations.

• Confusing the radius of a planet with the distance from its center

  Why it happens: Students sometimes use the radius of a planet as the distance r in the formula, instead of the distance from the center of the planet to the point where the field strength is being calculated.

  Fix: When calculating gravitational field strength at a point outside the surface of a planet, add the altitude to the radius of the planet to get the total distance r from the center. For example, if you are 1,000 km above the surface of a planet with a radius of 5,000 km, r = 6,000 km.

Where the marks go

The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.

Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.

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