Gravity Basics
What is gravity?
A pulling force that attracts all objects with mass toward each other.
What is the difference between mass and weight?
Mass = amount of matter (never changes, kg). Weight = gravitational force on that mass (changes by location, measured in N)
An orbit is the result of two things working together. Name both.
Gravity (pulling inward) AND sideways inertia (keeping the object moving forward).
Name Kepler's 1st Law and describe what it says in one sentence.
Law of Ellipses — every planet's orbit is an ellipse with the Sun at one focus.
An object weighs 588 N on Earth (g = 9.8). What is its mass?
m = W ÷ g = 588 ÷ 9.8 = 60 kg
In the formula F = Gm₁m₂/r², what does r stand for?
Distance between the centers of two objects
What unit is weight measured in?
Newtons (N)
What would happen to a satellite if it suddenly slowed down?
Gravity would win — the satellite would spiral inward toward Earth (and could crash).
What shape are all planetary orbits?
An ellipse (not a perfect circle!).
The force between two objects is 400 N. Both masses are doubled. Distance stays the same. New force?
400 × (2 × 2) = 400 × 4 = 1,600 N
If the distance between two objects doubles, the gravitational force becomes what fraction of the original?
One-fourth (¼) — because distance is squared in the denominator. (Inverse Square Law)
A student has a mass of 70 kg. What is her mass on the Moon?
70 kg — mass never changes, no matter where you are!
* DAILY DOUBLE *
Why do astronauts on the ISS appear to float even though gravity is pulling on them?
They are in free fall — they and the ISS fall together, so there is no normal force.
In Kepler's 3rd Law, what do T and a stand for?
T = orbital period (in Earth years). a = semi-major axis (in AU).
A planet's semi-major axis is 9 AU. Use T² = a³ to find its orbital period.
T² = 9³ = 729 → T = √729 = 27 years
Name the two things that affect the size of the gravitational force between two objects.
Mass of the objects AND the distance between them.
Write the formula used to calculate weight.
W = m × g
What is the name of the point in an orbit where a planet is CLOSEST to the Sun?
Perihelion
An orbit has an eccentricity of 0. What shape is it?
A perfect circle (eccentricity 0 = no stretching).
A rover weighs 1,960 N on Earth. What is its weight on the Moon? (g_Moon = 1.6)
Step 1: mass = 1960 ÷ 9.8 = 200 kg. Step 2: W_Moon = 200 × 1.6 = 320 N
The gravitational force between two planets is 500 N. The distance is tripled. What is the new force?
500 ÷ 9 = 55.6 N (distance ×3 → force ÷ 3² = ÷9)
A probe has a mass of 200 kg. What is its weight on Mars? (g = 3.7 m/s²)
W = 200 × 3.7 = 740 N
At perihelion, is a planet moving faster or slower than at aphelion? Use Kepler's 2nd Law to explain.
Faster — equal areas must be swept in equal times, so near the Sun the planet covers a wider arc in the same time.
Use T² = a³. A planet is 4 AU from the Sun. What is its orbital period?
T² = 4³ = 64 → T = √64 = 8 years
Two stars have a force of 3,600 N. If the distance is halved, what is the new force?
Distance ÷2 → force ×4: 3,600 × 4 = 14,400 N