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1.4.7Motion, Forces and Energy

Friction — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 9 parts Lesson 37 of 91 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

1.4.7 Friction — IGCSE PHYSICS notes: Describe friction and air resistance as forces that oppose motion, how they are changed and how they affect resultant forces.Friction = a force between two surfaces in contact that opposes the motion, or tendency to move, of one surface over …

  • Static friction
  • Sliding friction
  • Terminal velocity (Extended)
  • Steady speed
  • Streamlined
  • Proportional

Part 1 of 7What friction is

Friction
Friction a force between two surfaces in contact that opposes the motion, or tendency to move, of one surface over the other.
It acts along the surfaces in the direction opposite to the relative sliding.
Cause
  • Cause tiny bumps on even smooth-feeling surfaces catch each other.
Friction produces heating.

Part 2 of 7Useful and harmful friction

Useful
  • Useful friction lets us walk, grip a pencil and gives tyres grip to accelerate, brake and turn.
Harmful
  • Harmful it wastes energy as heat and wears moving parts in engines and bearings.
Rubbing hands together warms them because kinetic energy is converted to thermal energy.

Part 3 of 7Changing the size of friction

Changing the size of friction
Reduce friction:
  • Use lubricants such as oil, grease or water to separate the surfaces.
  • Use smoother surfaces.
  • Use wheels, ball bearings and rollers so that rolling replaces sliding.
Increase friction:
  • Use rougher surfaces and rubber treads on tyres or shoe soles.
  • Increase the force pressing the surfaces together, as a heavier box is harder to drag.
Contact area has little effect on friction for the same pair of surfaces.

Part 4 of 7Static and sliding friction

Static friction
  • Static friction before sliding, it matches the pushing force up to a maximum value.
Pushing a heavy crate with 50 N may not move it because static friction is also 50 N.
Sliding friction
  • Sliding friction once the push exceeds the maximum, the crate slides and friction is usually slightly smaller.
It is easier to keep a crate moving than to start it moving.

Part 5 of 7Air resistance and drag

1Drag
Drag the resistive force on an object moving through a fluid; in air it is called air resistance.
2
Drag increases with speed and with cross-sectional area.
3
A streamlined shape reduces drag, as shown by crouching cyclists and smooth car designs.
4Terminal velocity (Extended)
  • Terminal velocity (Extended) in free fall, air resistance rises until it equals the weight, so the resultant force is zero and the speed is constant.

Part 6 of 7Friction and the resultant force

Friction and the resultant force
Always include friction in the force diagram of a moving object.
Resultant force
Resultant force driving force − friction (and drag), then apply F = ma for the acceleration.
Steady speed
  • Steady speed the driving force equals the total resistive force.
Braking
  • Braking friction between brake pads and discs and between tyres and road slows the car and converts kinetic energy to heat.
Measuring sliding friction
  • Measuring sliding friction drag a block at steady speed with a force meter;
    • the reading equals the sliding friction.
  • Adding masses gives a straight line through the origin for friction against load.

Part 7 of 7More from the specification

1Frictional Forces and Energy Dissipation
  • Friction is a resistive force that opposes the relative motion of solid surfaces in contact, converting mechanical kinetic energy into thermal energy.
2Hooke's Law and Linear Extension
  • Hooke's law states that tensile extension is directly proportional to applied load (F = kΔx) until the limit of proportionality is reached on the force-extension graph.
3Centripetal Force in Circular Paths
  • When an object follows a circular path, friction, tension,
  • or gravity provides the inward centripetal force necessary to continuously alter the direction of velocity.
4Equilibrium of Collinear and Coplanar Forces
  • An object remains at rest or travels at constant velocity when the vector sum of all acting coplanar forces equals zero newtons, satisfying static equilibrium.
5Extension
  • Increase in length over the original length.
6Length
  • Resistance is directly proportional to the length of a wire.
7Newton
  • Unit of force, kg m s-2.
8Proportional
  • Changing by the same factor as another quantity.
9Balanced
  • Having equal and opposite moments.
10Elastic
  • Returns to original shape when force is removed.
11Spring constant
  • Stiffness k in N/m.
12Circular motion
  • Motion along a circular path with a force towards the centre.

Key terms

TermMeaning
FrictionForce opposing relative motion between surfaces in contact.
LubricantSubstance that reduces friction.
Static frictionFriction on an object that is not yet sliding.
DragResistive force on an object moving through a fluid.
StreamlinedShaped to reduce drag.

Avoid these mistakes

1
Thinking friction acts only when objects slide — static friction acts before sliding starts.
2
Saying friction acts in the direction of motion — it opposes the relative sliding of the surfaces.
3
Believing friction depends strongly on contact area — it does not for the same surfaces.
4
Saying friction is always bad — we could not walk or brake without it.

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