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Theme A — Space, Time and Motion

A.2.6 Non-Contact Forces — IB Physics HL Revision Notes & Topical Questions

5 min read 9 parts Lesson 17 of 63 1 quiz

📚 IB Physics HL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Lesson summary

A.2.6 Non-Contact Forces — IB PHYSICS SL notes: Describe gravitational, electric and magnetic forces as non-contact forces and use g = F/m as the gravitational field strength.Non-contact force = a force that acts at a distance, without physical contact between bodies, due to the …

  • Three forces in this course
  • Field idea
  • Nuclear forces
  • Gravitational force Fg
  • Mass and weight
  • Inverse-square law

Part 1 of 7Forces at a distance

1Non-contact force
Non-contact force a force that acts at a distance, without physical contact between bodies, due to the action of a field.
2Three forces in this course
  • Three forces in this course gravitational, electrostatic and magnetic.
3Field idea
  • Field idea an object creates a field in the space around it and any suitable object in the field feels a force.
4Nuclear forces
  • Nuclear forces the strong and weak forces also act at a distance, but only over extremely short ranges.

Part 2 of 7Gravitational force and weight

Gravitational force and weight
Gravitational force Fg
  • Gravitational force Fg the attractive force between two objects with mass; it is always attractive.
  • The force between a planet and a comet is an example.
  • It is extremely weak unless one of the bodies is as massive as a planet.
Weight
  • Weight on Earth, the gravitational force of the Earth on an object, Fg = mg.
  • It acts at the centre of mass and points towards the centre of the planet.
Gravitational field strength
  • Gravitational field strength g = F/m, in N kg−1, equivalent to m s−2.
  • On Earth g ≈ 9.8 N kg−1 and on the Moon about 1.6 N kg−1.
Mass and weight
  • Mass and weight an astronaut's mass stays the same on the Moon but the weight is about one-sixth as large.
Inverse-square law
  • F = Gm1m2/r2 is treated in the gravitation topic.

Part 3 of 7Electrostatic force

Electrostatic force Fe
  • Electrostatic force Fe the force on charged objects in an electric field; it can be attractive or repulsive.
  • A proton and an electron attract each other.
Rule
  • Rule like charges repel and unlike charges attract.
Field strength
  • Field strength a charge q in field E feels F = qE, so E is force per unit positive charge in N C−1.
Coulomb's law
  • F = kq1q2/r2.
    What the symbols mean: F = electric force (N) · Q1, Q2 = charges (C) · r = separation (m) · k = 1/4πε0 = 8.99 × 109 N m2 C−2
Strength
  • Strength the electric repulsion between two protons is about 1036 times their gravitational attraction.
  • A rubbed comb lifts paper against the pull of the whole Earth.
Origin of contact forces
  • Origin of contact forces electric forces underlie friction and the normal reaction.

Part 4 of 7Magnetic force

Magnetic force Fm
  • Magnetic force Fm the force between magnetic poles in a magnetic field; it can be attractive or repulsive.
  • The north and south poles of magnets attract.
Moving charge
  • Moving charge a charge q moving at speed v perpendicular to a field B feels F = qvB.
  • The force is perpendicular to both the velocity and the field.
No work
  • No work the force changes only the direction of the charge, not its speed, so the path is a circle.
Field lines
  • Field lines they run from north to south outside a magnet and are closer where the field is stronger.

Part 5 of 7Comparing the forces

FeatureGravitationalElectrostaticMagnetic
Acts onMassChargeMoving charge, current or magnet
Attract or repelAttract onlyBothBoth
Relative strengthWeakestStrongDepends on motion
FieldGravitational fieldElectric fieldMagnetic field
  • Large scales
    • gravity dominates because astronomical bodies are almost electrically neutral and mass always adds.

Part 6 of 7Using g

Uniform field
  • Uniform field any mass in a uniform field accelerates at g if gravity is the only force.
  • All objects fall together in a vacuum.
Changing g
  • Changing g weight changes on a high mountain or another planet while mass does not.
Measuring
  • Measuring a balance compares masses, while a newton meter or spring balance reads weight.
Room scale
  • Room scale the field is uniform, but over thousands of kilometres it falls with the square of the distance from the Earth's centre.
Free-body diagrams
  • Free-body diagrams include weight as a non-contact force.

Part 7 of 7More from the specification

More from the specification
Newton's Second Law and Momentum
  • Newton's second law states that the resultant force acting on a body is equal to its rate of change of linear momentum, represented by the vector equation Fnet = Δp / Δt = m·a for constant mass.
Centripetal Force in Circular Motion
  • An object moving in a circle at constant speed experiences an inward centripetal acceleration a = v²/r = ω²r;
    • a continuous resultant centripetal force of magnitude F = m·v²/r = m·ω²r acts perpendicular to velocity, altering direction without changing speed.
Fluid Resistance and Terminal Velocity
  • When a body moves through a fluid, opposing viscous drag forces increase with speed until the resultant force becomes zero,
  • producing a constant terminal velocity.
Tension
  • Force transmitted along a string or rope.
Buoyancy
  • Another name for upthrust.
Centripetal force
  • Net force toward the centre, mv2/r.
Kinetic energy
  • K = ½mv2, energy of motion.
Angular speed
  • Magnitude of the angular velocity.
Centripetal acceleration
  • Ac = v2/r toward the centre.
Fluid resistance
  • Drag force from a fluid opposing motion.
Momentum
  • P = mv, a vector.
Resultant
  • Vector sum of two or more vectors.
Constant
  • Not changing.
Collision
  • Short interaction with large internal forces.
Linear
  • A graph that is a straight line through the origin, so resistance is constant.
Impulse
  • J = FΔt = Δp.
    What the symbols mean: F = force (N) · Δt = time the force acts (s) · Δp = change in momentum (kg m s−1); FΔt is the impulse (N s)
Radius
  • Distance from the centre to the circular path.

Key terms

TermMeaning
Non-contact forceForce acting at a distance through a field, with no physical contact
Gravitational field strengthForce per unit mass, g = F/m
WeightGravitational force of a planet on an object, mg
Electrostatic forceForce between charged objects, attractive or repulsive
Magnetic forceForce on moving charges, currents and magnets in a magnetic field
FieldRegion in which an object experiences a force at a distance

Avoid these mistakes

1
Believing mass changes with location — mass is constant while weight depends on g.
2
Thinking gravity needs a medium — it acts through a vacuum.
3
Saying gravity is the strongest force at atomic scale — it is by far the weakest.
4
Thinking a magnetic force changes the speed of a charge — it only changes the direction.
5
Forgetting that the Earth is also pulled by the object — the forces are a third-law pair.

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