A man and his friend have masses 70kg and 60kg respectively. If they are seated 1.0m apart, calculate the…

Analyze the question We need the gravitational force between two masses 1.0 m apart. This is Newton's law of universal gravitation.

Concept F = G·m₁·m₂ / r²

where G = 6.67 × 10⁻¹¹ N m² kg⁻², m₁ = 70 kg, m₂ = 60 kg, r = 1.0 m.

Solution

F = (6.67 × 10⁻¹¹ × 70 × 60) / (1.0)²

70 × 60 = 4200

F = 6.67 × 10⁻¹¹ × 4200 F = 28014 × 10⁻¹¹ F = 2.80 × 10⁻⁷ N

Check against the options

• A (1.59 × 10⁻¹⁴ N): too small
• B (2.80 × 10⁻⁷ N): matches our result
• C (1.30 × 10⁻²N): too large
• D (6.30 × 10¹³ N): far too large

Tip For gravitation questions, G is tiny (10⁻¹¹), so for everyday objects like people, the force will be extremely small, around 10⁻⁷ N or less. This helps you quickly eliminate options that are huge. Also remember to square the distance, and to multiply the masses, not add them.