Physics
Gravitational & electric fields — quick study summary
Fields describe forces at a distance. Gravitational field strength g = F/m (force per unit mass); near Earth's surface, g ≈ 9.81 m/s². Newton's law of gravitation: F = Gm₁m₂/r². Electric field strength E = F/q; Coulomb's law: F = kq₁q₂/r². Both forces drop with 1/r² and are vector fields — direction matters. Gravity always attracts; electric forces can attract or repel depending on charge signs.
Key points
- Gravitational: F = Gm₁m₂/r²; G = 6.67 × 10⁻¹¹ N·m²/kg²
- Electric: F = kq₁q₂/r²; k ≈ 9 × 10⁹ N·m²/C²
- Field strength = force per unit mass (g) or per unit charge (E)
- Both forces follow inverse-square: double r → quarter the force
- Gravity only attracts; electric can attract OR repel
Practice quiz
Click each question to reveal the answer.
1. If you double the distance between two masses, what happens to the gravitational force?
- Halved
- 1/4
- Doubled
- Unchanged
Answer: It becomes 1/4 of the original
F ∝ 1/r². Double r → 1/(2²) = 1/4 the force.
2. What is the electric field strength at a point where a 2 C charge feels a 10 N force?
Answer: 5 N/C
E = F/q = 10 ÷ 2 = 5 N/C.
3. Why is gravitational force always attractive but electric force can be repulsive?
Answer: Mass is always positive; charge can be positive or negative, so like charges repel
There's no 'negative mass' — gravity only pulls. Electric charges come in two signs, so opposite-signs attract and like-signs repel.
Last reviewed: May 2026