6.1.5Space Physics
Gravitational Field Strength — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions
5 min read 10 parts Lesson 10 of 24 1 quiz
Curriculum Equation Key Circular Motion & Gravitation LawsFast Exam Solution
a_c = v² / r | F_c = (m × v²) / r | F_g = G × (m₁ × m₂) / r² | v_orbit = √(G×M / r)
Lesson summary
6.1.5 Gravitational Field Strength — IGCSE Physics Notes — IGCSE PHYSICS notes: Define gravitational field strength, use g = W/m, and describe how it depends on mass and distance.Gravity — every object with mass attracts every other object, and the force is noticeable only when o…
- Definition
- Mass of the planet
- Radius of the planet
- At a surface
- On the Moon
- Far from large masses
Part 1 of 8Gravity and the gravitational field
Gravity
- Gravity every object with mass attracts every other object, and the force is noticeable only when one mass is huge, like a planet.
Field
- Field the region around a mass in which another mass feels a gravitational force is its gravitational field.
Direction
- Direction the force always pulls the two masses towards each other.
Part 2 of 8Gravitational field strength
Definition
- Definition g is the gravitational force acting per unit mass on an object placed in the field.
Equation
- Equation g = W / m, or W = m g.
- W is the weight in newtons (N).
- m is the mass in kilograms (kg).
- g is the gravitational field strength in newtons per kilogram (N/kg).
Free fall
- Free fall the unit N/kg is the same as m/s², so g is also the acceleration of free fall.
Part 3 of 8What g depends on
Mass of the planet
- Mass of the planet the greater the mass, the greater the field strength at its surface.
Radius of the planet
- Radius of the planet g at the surface also depends on how far the surface is from the centre.
Distance
- Distance as the distance from a planet increases, the field strength decreases.
At a surface
- At a surface the value of g is constant over small changes of height.
Part 4 of 8Values of g
| Body | g (N/kg) |
|---|---|
| Moon | 1.6 |
| Mercury | 3.7 |
| Mars | 3.7 |
| Venus | 8.9 |
| Saturn | 9.0 |
| Earth | 9.8 |
| Neptune | 11.0 |
| Jupiter | 23.1 |
Earth
- Earth a 1 kg mass is pulled with a force of 9.8 N, often taken as 10 N/kg.
Moon
- Moon g is about one-sixth of its value on the Earth.
Part 5 of 8Mass and weight
Mass
- Mass the amount of matter in an object, measured in kg and the same everywhere.
Weight
- Weight the gravitational force on an object, measured in N, which depends on the local value of g.
On the Moon
- On the Moon mass stays the same but weight falls to about one-sixth.
Far from large masses
- Far from large masses weight is almost zero but mass is unchanged.
Measurement
- Measurement weight is measured with a newton-meter, mass with a balance.
Part 6 of 8Free fall
Acceleration
- Acceleration if weight is the only force, the object accelerates at g.
Equal for all masses
- Equal for all masses in a vacuum all objects fall with the same acceleration, because a = F / m = mg / m = g.
Air resistance
- Air resistance on Earth a feather falls more slowly than a hammer because of drag, not gravity.
Part 7 of 8Gravity and orbits
Orbit force
- Orbit force the gravitational attraction of the Sun acts towards its centre and keeps the planets in orbit.
Sun's mass
- Sun's mass the Sun holds over 99% of the mass of the Solar System, so it dominates the orbits.
Further out
- Further out as the distance from the Sun increases the Sun's field strength decreases.
Effect on orbits
- Effect on orbits planets further out have smaller orbital speeds and longer orbital periods.
Part 8 of 8More from the specification
1Gravitational Field Strength
- The gravitational force exerted per unit mass (g = F / m) governs the orbital trajectories of moons, planets,
- and comets around massive astronomical bodies.
2Comet Orbits and Energy Conservation
- A comet follows a highly eccentric elliptical orbit, accelerating and gaining kinetic energy as it converts gravitational potential energy near perihelion.
3Earth's Tilt and Daylight Hours
- The 23.5-degree axial tilt of Earth causes the northern hemisphere to tilt toward the Sun during summer,
- resulting in longer hours of daylight and shorter night periods.
4Hemispheric Seasonal Solar Insolation
- Summer occurs in a hemisphere when incoming solar light strikes the surface at a more perpendicular angle, concentrating solar energy over extended daylight hours.
5Hemisphere
- Half of the Earth, northern or southern.
6Nebula
- Cloud of gas and dust in which stars form.
7Comet
- An icy body on a highly elliptical orbit around the Sun.
8Elliptical orbit
- Orbit with constant E and L but varying speed.
9Kinetic energy
- K = ½mv2, energy of motion.
10Artificial satellite
- A man-made object orbiting a body in space.
11Gravitational potential
- Work done per unit mass in bringing a test mass from infinity to the point.
12Potential energy
- Energy stored by the configuration of a system.
Key terms
| Term | Meaning |
|---|---|
| Gravitational field strength | Force per unit mass at a point in a gravitational field, in N/kg. |
| Weight | The gravitational force on an object, W = m g, in newtons. |
| Mass | The amount of matter in an object, in kg, the same everywhere. |
| Free fall | Falling under gravity alone, with acceleration g. |
| Gravitational field | The region around a mass where another mass experiences a force. |
Avoid these mistakes
1
Using kg for weight or N for mass — weight is a force in N and mass is in kg.
2
Saying mass changes on the Moon — only the weight changes.
3
Saying heavier objects fall faster in a vacuum — all fall with the same acceleration.
4
Giving the unit of g as N or kg — it is N/kg (or m/s²).
5
Saying there is no gravity in space — gravity acts everywhere and is weaker far from a large mass.
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