A gravitational field may be represented by lines of gravitational force. State what is meant by a line of gravitational force.

Mark scheme

Marking points from the official mark scheme (May/June 2018). The mark or mark type is shown in brackets.

1. [B1] direction of force on a (small test) mass or path in which a (small test) mass will move
2. [B1] (at surface,) lines (of force) are radial
3. [B1] Earth has large radius/height above surface is small so lines are (approximately) parallel
4. [B1] parallel lines → constant field strength
5. [C1] (change in) KE of rock = (change in) PE or ½mv² = GMm/R
6. [C1] (m)v² = (m)(2 × 6.67 × 10⁻¹¹ × 7.4 × 10²²)/ (1.7 × 10³ × 10³)
7. [A1] v = 2.4 × 10³ m s⁻¹
8. [B1] correct conclusion based on comparison of v with 2.8 km s⁻¹
9. (or)
10. [C1] (change in) KE of rock = (change in) PE
11. [C1] (at infinity) Ep = (6.67 × 10⁻¹¹ × 7.4 × 10²² × m) / (1.7 × 10³ × 10³) = 2.9 × 10⁶ m
12. [A1] Ek of rock = ½ × m × (2.8 × 10³)² = 3.9 × 10⁶ m
13. [B1] correct conclusion based on comparison of EK and Ep values
14. (or)
15. [C1] (change in) KE of rock = (change in) PE or ½mv² = GMm/R
16. [C1] (m) (2800)² = (m) (2 × 6.67 × 10⁻¹¹ × 7.4 × 10²²)/R
17. [A1] R = 1.3 × 10³ km
18. [B1] correct conclusion based on comparison of R with 1.7 × 10³ km
19. (or)
20. [C1] (change in) KE of rock = (change in) PE or ½mv² = GMm/R
21. [C1] (m) (2800)² = (m) (2 × 6.67 × 10⁻¹¹ × M) / (1.7 × 10⁶)
22. [A1] M = 1.0 × 10²³ kg
23. [B1] correct conclusion based on comparison of M with 7.4 × 10²² kg