Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

Theme A — Space, Time and Motion

A.2.5 Contact Forces — IB Physics HL Revision Notes & Topical Questions

5 min read 10 parts Lesson 16 of 63 1 quiz

📚 IB Physics HL Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
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.
0%read
Lesson summary

A.2.5 Contact Forces — IB PHYSICS SL notes: Describe the normal reaction force, tension, surface friction and viscous drag as contact forces and calculate them in simple situations.Contact force = a force that acts between objects that are physically touching.Examples — friction,…

  • Ending contact
  • Surface friction Ff
  • Viscous drag Fd
  • Normal force FN
  • Response force
  • Horizontal surface, no other vertical forces

Part 1 of 8What a contact force is

Contact force
Contact force a force that acts between objects that are physically touching.
Examples
  • Examples friction, fluid resistance (viscous drag), tension, the normal reaction, upthrust and the force from a compressed spring.
Origin
  • Origin at the atomic scale they arise from electromagnetic repulsion between the atoms of the surfaces.
  • A solid surface deforms slightly like a very stiff spring and its elastic response pushes back.
Ending contact
  • Ending contact the force disappears as soon as contact ends, unlike weight, which acts at a distance.
Third law
  • Third law contact forces between two objects are equal, opposite and of the same kind.

Part 2 of 8Surface friction and viscous drag

Surface friction Ff
  • Surface friction Ff a force that opposes motion when the surfaces of objects rub against each other.
  • Car wheels on the ground are an example.
Viscous drag Fd
  • Viscous drag Fd a type of friction that acts when an object moves through a fluid.
    • (a liquid or a gas)
  • Air resistance is viscous drag in air.

Part 3 of 8Normal reaction force

Normal reaction force
Normal force FN
  • Normal force FN the component of the contact force perpendicular to the surface that supports a body.
  • A book on a table is supported by it.
Response force
  • Response force it adjusts automatically to the size needed to stop objects passing into one another.
Horizontal surface, no other vertical forces
  • FN = mg.
Slope of angle θ
  • FN = mg cos θ.
Pull upwards at an angle
  • Pull upwards at an angle it reduces the normal force.
Lift accelerating upwards at a
  • FN = m(g + a).
Never negative
  • Never negative a surface can push but cannot pull; a result below zero means the object has left the surface.

Part 4 of 8Tension and compression

1Tension FT
Tension FT the pulling force transmitted along a string, rope, cable or rod when forces pull on both ends.
  • A mass on a spring suspended from a clamp is an example.
2Light string
  • Light string the tension is the same at every point and a smooth pulley changes only its direction.
3Compression
  • Compression a string can only pull, while a rod or strut can also push.
4Vertical rope at rest
  • T = mg; if the mass accelerates upwards T = m(g + a).
5Rope at angle θ to the horizontal
  • Rope at angle θ to the horizontal horizontal component T cos θ and vertical component T sin θ.
6Pendulum at the lowest point
  • Pendulum at the lowest point the tension exceeds the weight because it also supplies the centripetal force.

Part 5 of 8Apparent weight

1Scales reading
  • Scales reading bathroom scales show the normal force, not the true weight.
2Lift at constant velocity
  • Lift at constant velocity the reading equals mg.
3Lift accelerating up or down
  • Lift accelerating up or down the reading is greater or less than mg.
4Free fall
  • Free fall the reading is zero, giving apparent weightlessness.
5Astronauts in orbit
  • Astronauts in orbit they and the spacecraft are in free fall, so no normal force acts between them and the floor.
6Newton meter
  • Newton meter placed in series with a string, it reads the tension.

Part 6 of 8Contact and non-contact forces together

1Child pulling a sledge uphill
  • Child pulling a sledge uphill tension in the rope, normal force from the ground, surface friction on sledge and shoes,
    • and drag from the air are contact forces.
2Weight
  • Weight the gravitational pull of the Earth on the child and the sledge is a non-contact force.

Part 7 of 8Solving problems with contact forces

1
Draw the free-body diagram with each contact force at the surface or string where it acts.
2
Resolve forces parallel and perpendicular to the surface.
3
Apply F = ma along each direction, with zero acceleration perpendicular to a fixed surface.
4
Check that normal forces and tensions are not negative.

Part 8 of 8More from the specification

1Normal Contact Force and Friction
  • Contact forces arise from microscopic electrostatic repulsion between surfaces;
    • frictional force acts parallel to interface surfaces opposing relative motion,
  • while normal contact force acts perpendicularly.
2Impulse-Momentum Relationship
  • The impulse delivered by a resultant contact force acting over a time interval delta_t equals the change in linear momentum: J = F delta_t = delta_p = m(v - u), measured in N s or kg m s−1.
3Conservation of Linear Momentum in Collisions
  • In any closed system where external forces sum to zero, total initial linear momentum equals total final linear momentum across elastic and inelastic collisions.
4Centripetal Contact Force in Circular Motion
  • For an object moving along a circular path of radius r at speed v, the resultant inward contact force provides the required centripetal acceleration: Fc = m v2 / r = m omega2 r.
5Buoyancy
  • Another name for upthrust.
6Static friction
  • Friction between surfaces at rest relative to each other; fs ≤ μsFN.
7Kinetic energy
  • K = ½mv2, energy of motion.
8Angular speed
  • Magnitude of the angular velocity.
9Linear speed
  • The distance travelled along the circle per unit time, v = ωr.
    What the symbols mean: v = speed (m s−1) · ω = angular speed (rad s−1) · r = radius (m) · T = period (s)
10Momentum
  • P = mv, a vector.
11Collision
  • Short interaction with large internal forces.
12Linear
  • A graph that is a straight line through the origin, so resistance is constant.
13Impulse
  • 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)
14Radius
  • Distance from the centre to the circular path.

Key terms

TermMeaning
Contact forceForce that acts between objects that are physically touching
Normal forceContact force perpendicular to a surface
TensionPulling force transmitted along a string, rope or rod
Surface frictionContact force opposing relative motion between touching surfaces
Viscous dragFrictional force opposing the motion of an object through a fluid
Apparent weightThe normal force that a supporting surface exerts on a body

Avoid these mistakes

1
Treating the normal force as always equal to mg — it depends on the angle, other vertical forces and acceleration.
2
Thinking a surface can pull — a normal force only pushes.
3
Assuming tension changes along a light string over a smooth pulley — it stays the same.
4
Confusing true weight with scale reading — scales show the normal force.
5
Treating contact forces as continuing after contact ends — they stop immediately.

Last updated:

\n