Friction — Types and Effects
At the 2023 Bathurst 1000, Supercars teams spent over $80,000 per car on tyres — every tread choice a calculated bet on friction to shave seconds off lap times.
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Q1 · Try sliding a book across a smooth desk, then put sandpaper under it. What do you notice?
Q2 · Name two situations where friction helps you and two where it causes a problem.
● Know
- Friction is a contact force opposing motion between surfaces
- The four types: static, sliding (kinetic), rolling, fluid
- Examples of useful and harmful friction
● Understand
- Why friction always acts opposite to the direction of motion
- How surface roughness and pressing force affect friction
- Why rolling friction is less than sliding friction
● Can do
- Identify the type of friction in a given situation
- Describe methods to increase or decrease friction
- Design a fair test comparing friction on different surfaces
Every time you write with a pencil, something is grinding pencil graphite onto paper. That's friction — and without it, your pencil would slide right off the page.
Friction is a contact force that opposes motion (or attempted motion) between two surfaces. Key facts:
- Friction always acts in the opposite direction to motion — if you push a box forward, friction acts backward
- Even "smooth" surfaces have microscopic bumps that catch and drag on each other
- Friction converts kinetic energy into heat — that's why your hands warm up when you rub them
Two main factors affect how much friction exists between surfaces:
- Surface roughness — rougher surfaces have more bumps, more friction
- Normal force — the harder the surfaces press together, the more friction
| Type | When it acts | Example | Relative size |
|---|---|---|---|
| Static | Before motion begins — prevents sliding | Book sitting on a tilted board before it slides; your shoes on the floor before you move | Largest |
| Sliding (kinetic) | While two surfaces slide past each other | A book sliding across a table; chalk dragging on a board | Less than static |
| Rolling | When one object rolls over another | A bike wheel on the road; a ball rolling across grass | Much less than sliding |
| Fluid (drag) | When moving through a liquid or gas | A swimmer in water; a car moving through air; a skydiver falling | Depends on speed and shape |
This is why wheels were invented: rolling friction is much less than sliding friction. Moving a heavy box on wheels takes far less force than dragging it.
Reducing friction: lubricants (oil, wax), smooth surfaces, ball bearings, streamlining (for fluid drag).
Increasing friction: grip tape, rough tyre tread, cleats on boots, roughened surfaces.
Useful friction — we'd be helpless without it:
- Car tyres gripping the road — at the Bathurst 1000 (Mount Panorama), tyre compound choice is critical. Hard tyres last longer but give less grip; soft tyres give more grip but wear out faster. Engineers balance these for each stint.
- Safety boots on construction sites — Australian workplace safety requires grip
- Writing and drawing — pencil graphite needs friction to stick to paper
- Brakes — brake pads press against discs, sliding friction converts kinetic energy to heat, slowing the car
- Climbing ropes — friction between the rope and rock anchor holds a climber's weight
Harmful friction — wastes energy and causes wear:
- Engine wear — moving metal parts grind against each other; engine oil is essential to reduce this
- Brake pad wear — brake pads gradually erode away (the energy goes into heat and sound)
- Energy loss in machines — about 20% of fuel energy in a car is lost to friction in the engine and drivetrain
- Bushfire risk — sparks from machinery friction can ignite dry Australian bush; fire risk protocols include checking for friction-generating faults
A student pushes a heavy box across the floor. They say: "Once the box is moving, it should take LESS force to keep it going than to start it." Are they right? Predict which type of friction is larger — static or sliding — and explain why.
How close was your prediction?
Correct — static friction is always the largest friction type between two surfaces.
Remember: it takes MORE force to START motion than to KEEP motion going.
In each set below, three share the same type of friction and one does not. Identify the odd one out and explain why.
Set 1: A book sitting still on a tilted board · A car parked on a hill · A foot not yet sliding on a wet floor · A ball rolling down a ramp
Set 2: A swimmer in a pool · A skydiver falling · A car driving through rain · A book sliding on a desk
Set 3: Engine oil lubricating pistons · Wax on a surfboard · Adding grip tape · Grease on a bicycle chain
Set 4: A hockey puck sliding on ice · A book sliding off a table · A ball rolling across carpet · Chalk scraping a blackboard
Design a fair test to compare friction on 3 different surfaces (e.g. wood, sandpaper, carpet). Your plan must include:
- Aim: What question are you trying to answer?
- Independent variable (IV): What one thing are you changing?
- Dependent variable (DV): What are you measuring?
- Controlled variables (at least 2): What stays the same to make the test fair?
- Method: Briefly describe the steps
- Expected result: Which surface do you predict has the most friction?
Q1. Identify the type of friction in each situation: (a) A book sitting still on a tilted board. (b) A hockey puck sliding on ice. (c) A ball rolling across grass. (d) A swimmer moving through water. (4 marks)
Q2. Explain two ways friction is useful in everyday life and one way it is harmful. Use specific examples. (3 marks)
Q3. A student tests three surfaces (wood, sandpaper, carpet) to see which has the most friction. Identify the IV, DV and two controlled variables. (3 marks)
Answers
▾MCQ 1
B — Friction always acts opposite to the direction of motion (or attempted motion). This is why it slows things down or prevents movement from starting.
MCQ 2
C — Static friction acts before motion starts. It is what keeps stationary objects from sliding. Once the object begins moving, the friction type changes to sliding (kinetic) friction.
MCQ 3
B — A lubricant fills the microscopic gaps between surfaces, separating the bumps and allowing surfaces to slide more easily. Grip tape, roughening, and increasing weight all increase friction.
MCQ 4
C — Air resistance is friction from a fluid (in this case air, which is a gas). It acts on any object moving through air and always opposes the direction of motion.
MCQ 5
C — Deeper tread pattern means more rubber-to-road contact and more bumps gripping the surface, increasing friction (traction). Inflating more, adding lubricant, or making tyres narrower all reduce traction.
Short Answer 1
Model answer: (a) Static friction — the book is not yet moving, static friction prevents it from sliding. (b) Sliding (kinetic) friction — the puck is sliding across the ice surface. (c) Rolling friction — the ball rolls, meaning its surface rotates against the grass rather than sliding. (d) Fluid friction (drag) — the swimmer moves through water, which is a fluid that resists their motion.
Short Answer 2
Model answer: Useful: (1) Car brakes — brake pads press against spinning discs, sliding friction converts kinetic energy to heat, slowing the car safely. (2) Shoe soles on floors — static friction between shoe and ground prevents slipping when walking. Harmful: Engine wear — moving metal parts in a car engine rub against each other, causing wear and energy loss as heat. Engine oil is needed to reduce this harmful friction.
Short Answer 3
Model answer: IV (Independent Variable): the type of surface (wood, sandpaper, carpet) — this is what the student changes. DV (Dependent Variable): the friction force, measured as the force needed to slide an object across the surface (e.g. using a Newton meter). Controlled variables (any two): the mass of the object being slid; the same object being used each time; the same speed of pulling; the same area of contact between object and surface.
At the start of this lesson you were asked why racing teams at Bathurst choose different tyre compounds for different parts of the track. Now you know the answer — it's all about controlling friction!
Explain which type of friction is most important when a race car corners hard at Bathurst, and why softer tyre compounds create more friction. Then revisit your original examples of helpful and harmful friction, adding the correct friction type (static, sliding, rolling, or fluid) for each.
- Friction is a contact force opposing motion — always acts opposite to direction of movement. Four types: static, sliding, rolling, fluid.
- Static > sliding > rolling. Fluid friction depends on speed and shape. Lubricants reduce friction; rough surfaces increase it.
- Friction is both useful (brakes, walking, tyres) and harmful (engine wear, energy loss) — engineers like those at Bathurst design around it.