Question 1

What is the gravitational constant G?

Accepted Answer

G = 6.674 × 10⁻¹¹ N·m²/kg² is a fundamental constant of nature that appears in Newton's law of gravitation. It was first measured by Henry Cavendish in 1798 using a torsion balance — an extraordinarily precise experiment for its era. G is the least precisely known of the fundamental constants; current measurements have a relative uncertainty of about 22 parts per million. Its small value explains why gravity between everyday objects is negligible.

Question 2

Why is gravity the weakest fundamental force?

Accepted Answer

Of the four fundamental forces — gravity, electromagnetism, strong nuclear, and weak nuclear — gravity is by far the weakest. The electromagnetic force between a proton and electron is about 10³⁶ times stronger than gravity between them. Gravity dominates at large scales only because it is always attractive (never repulsive) and has infinite range, so it accumulates with the enormous masses of stars and planets. Electromagnetism at large scales cancels because matter is electrically neutral overall.

Question 3

How does gravitational force vary with altitude?

Accepted Answer

Gravitational force decreases with altitude because distance r from Earth's center increases. At altitude h above the surface, r = R_Earth + h, so g(h) = GM/(R+h)² = g_surface × (R/(R+h))². At 400 km altitude (ISS orbit), g ≈ 8.7 m/s² — about 89% of surface gravity. Astronauts are "weightless" not because gravity is gone, but because they are in free fall: both they and the station fall at the same rate, eliminating the contact force (apparent weight).

Question 4

What is the difference between gravitational force and gravitational field strength?

Accepted Answer

Gravitational force F = Gm₁m₂/r² is the actual force between two specific masses. Gravitational field strength g at a point is defined as the force per unit mass that would be experienced by a small test mass placed there: g = F/m = GM_source/r². Field strength is a property of the space created by a mass, independent of what test mass you place there. Units: g has units N/kg = m/s², while F has units Newtons.

Question 5

How does weight differ from gravitational force?

Accepted Answer

They are the same thing — weight IS the gravitational force on an object: W = mg, where g is the local gravitational field strength. However, what a scale measures is the normal force (contact force), not gravity directly. In an elevator accelerating upward, you feel heavier (scale reads more) because the normal force increases, even though gravity is unchanged. In free fall, the scale reads zero (apparent weightlessness) but gravitational force is unchanged.

Question 6

What is orbital velocity and how does gravity determine it?

Accepted Answer

An orbit occurs when a satellite's centripetal acceleration exactly equals gravitational acceleration: v²/r = GM/r², giving orbital velocity v = √(GM/r). For a circular orbit at radius r, this is the exact speed needed to maintain the orbit. The ISS at 400 km altitude has v ≈ 7.66 km/s (27,600 km/h). Lower orbits require higher speeds; higher orbits require lower speeds. Geostationary orbit (v = Earth's rotation speed) occurs at r ≈ 42,164 km from Earth's center.

Planet	g (m/s²)	Relative to Earth	Weight of 70 kg (N)
Earth	9.81	1.00×	686.7 N
Moon	1.62	0.17×	113.4 N
Mars	3.72	0.38×	260.4 N
Jupiter	24.79	2.53×	1735.3 N
Saturn	10.44	1.06×	730.8 N
Venus	8.87	0.90×	620.9 N
Mercury	3.7	0.38×	259.0 N

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