CCEA A Level Physics
CCEA A Level Physics 5.1 Force Fields and 5.2 Gravitational Fields Exam Questions
Free CCEA A Level Physics revision for 5.1 Force Fields and 5.2 Gravitational Fields: what you need to know, common mistakes and exam-style questions with answers. Covers the field concept, field lines, gravitational field strength, newton's law of gravitation, orbits, kepler's third law and geostationary satellites.
Practise 5.1 Force Fields and 5.2 Gravitational Fields with past papers
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About CCEA A Level Physics 5.1 Force Fields and 5.2 Gravitational Fields
These sections open Unit A2 2: Fields, Capacitors and Particle Physics. Section 5.1 introduces a force field as a region where a body experiences a force, represented by field lines showing direction and strength. Section 5.2 applies this to gravity: gravitational field strength, Newton's law of universal gravitation, g = GM/r^2, planetary and satellite orbits, Kepler's third law and geostationary satellites. The ideas link back to circular motion and forward to electric fields, which you must compare with gravitational fields. Examiners like deriving Kepler's third law from equating forces, calculating the radius of a geostationary orbit and drawing field patterns.
What you need to know
- Force fields A force field is a region in which a body experiences a force. Field lines show the direction of the force on a test mass or charge, and closer lines mean a stronger field.
- Gravitational field strength g = F/m, the force per unit mass, unit N kg-1. Near the Earth's surface the field is approximately uniform; around a planet it is radial.
- Newton's law of gravitation F = GMm/r^2, where r is the distance between centres and G = 6.67 x 10^-11 N m^2 kg-2. The force is always attractive.
- Field of a planet g = GM/r^2 outside a spherical mass, so g follows an inverse square law with distance from the centre.
- Orbits and Kepler's third law For a circular orbit, gravitational force provides the centripetal force: GMm/r^2 = mrω^2 = m(4π^2/T^2)r, giving T^2 = (4π^2/GM) r^3, so T^2 is proportional to r^3.
- Geostationary satellites Orbit above the equator with a period of 24 hours in the same direction as the Earth rotates, so they stay above one point. Used for communications and broadcasting.
Common mistakes in 5.1 Force Fields and 5.2 Gravitational Fields
- Using the height above the surface as r; r is measured from the centre of the planet.
- Saying a satellite in orbit is weightless because there is no gravity; gravity provides the centripetal force.
- Describing a geostationary orbit without all three conditions: equatorial, 24-hour period and same direction of rotation as the Earth.
- Thinking satellite mass affects orbital period; it cancels when forces are equated.
Try three 5.1 Force Fields and 5.2 Gravitational Fields exam-style questions
1. Show that g at the Earth's surface is about 9.8 N kg-1. M = 5.97 x 10^24 kg, R = 6.37 x 10^6 m. (2 marks)
Show the mark scheme answer
g = GM/R^2 = 6.67 x 10^-11 x 5.97 x 10^24 / (6.37 x 10^6)^2; = 9.81 N kg-1.
2. Calculate the orbital radius of a geostationary satellite. (3 marks)
Show the mark scheme answer
r^3 = GMT^2/4π^2; = 6.67 x 10^-11 x 5.97 x 10^24 x (86 400)^2 / 39.5 = 7.5 x 10^22 m^3; r = 4.2 x 10^7 m.
3. Derive Kepler's third law for a circular orbit. (3 marks)
Show the mark scheme answer
Gravitational force provides centripetal force: GMm/r^2 = mω^2 r; ω = 2π/T so GM/r^2 = 4π^2 r/T^2; T^2 = (4π^2/GM) r^3, so T^2 is proportional to r^3.
5.1 Force Fields and 5.2 Gravitational Fields FAQs
What does CCEA A Level Physics 5.1 and 5.2 cover?
The idea of a force field and field lines, gravitational field strength, Newton's law of gravitation, g = GM/r^2, planetary and satellite orbits, Kepler's third law and geostationary satellites.
Which unit are 5.1 and 5.2 examined in?
Both are in Unit A2 2: Fields, Capacitors and Particle Physics.
How do I tackle orbit calculations?
Always start by writing gravitational force equals centripetal force, then cancel the satellite mass and rearrange for the quantity asked, keeping r measured from the planet's centre.
