State Newton's universal law of gravitation. Write its formula and explain each symbol. What is the value and SI unit of G?
How to lay out the answer. Follow the frame, then write it in your own words.
1 · State the law. Every particle in the universe attracts every other particle with a force directly proportional to the product of their masses and inversely proportional to the square of the distance between them.
2 · Write the formula. F = G m₁m₂/r²
3 · Explain the symbols.
F: gravitational force in newton
m₁, m₂: the two masses in kg
r: distance between their centres in m
G: universal gravitational constant
4 · Give G and its unit. G = 6.67 × 10⁻¹¹ N m² kg⁻². Its value is the same everywhere, so it is called universal.
5 · Real-life link. The same force makes fruit fall to the ground and holds the moon in its orbit.
6 · Close. So gravitation is a universal force that is always attractive.
Scoring tip: Get the unit of G right: N m² kg⁻². Say 'square of the distance' clearly.
