Year 7 Science · Unit 3 · Lesson 3
Challenge Worksheet
Learning Goals
1. Name the four types of friction. Give one everyday example of the type that acts in a liquid or a gas.
Find the error
Tom’s explanations
Line 1: Friction acts in the same direction as a sliding box moves, which is why the box slows down.
Line 2: Putting a heavy box on wheels reduces friction because the wheels make the box lighter.
Line 3: Of static, sliding and rolling friction, static friction is the largest.
Line 4: Rubbing your hands together warms them because friction destroys the energy of their motion.
2. Three of Tom’s four lines contain an error. Name each wrong line and rewrite it correctly.
Plan an investigation: which sole grips best?
Plan a fair test to find out which shoe grips a classroom floor best: a netball shoe, a leather school shoe or a rubber thong. You have the three shoes, a set of masses, a kitchen scale and a spring scale.
Hypothesis
Variables
Method
Results table
| Shoe | Trial 1 (N) | Trial 2 (N) | Average (N) |
|---|---|---|---|
| Netball shoe | |||
| Leather school shoe | |||
| Rubber thong |
What results would support your hypothesis?
Read the data: stopping on different roads
Claim: “More friction is always better for cars.”
A driving school tested the same car, with the same tyres, braking hard from 60 km/h on four road surfaces.
| Road surface | Dry asphalt | Wet asphalt | Loose gravel | Ice |
|---|---|---|---|---|
| Braking distance (m) | 20 | 35 | 40 | 140 |
Source: made-up but realistic results for this worksheet.
3. On which surface was there the most friction between the tyres and the road? Use Table 1 to explain how you know.
4. Evaluate the claim using Table 1 and what the lesson says about friction inside a car’s engine.
5. A council is building a steep concrete footpath up to a school. Engineer A says: “Make the surface as smooth as possible, so it is easy to clean.” Engineer B says: “Give it a rough surface.” Evaluate both suggestions, including what happens when it rains, and recommend one.
Question 1
Static, sliding (kinetic), rolling and fluid friction (1). Fluid friction example, e.g. a swimmer in water or air resistance on a car (1). [2 marks]
Find the error
2. Line 1: friction acts opposite to the motion (1). Line 2: wheels do not make it lighter; they swap sliding for much smaller rolling friction (1). Line 4: the energy is not destroyed; friction converts it into heat (1). Line 3 is correct. [3 marks]
Plan an investigation: which sole grips best?
Hypothesis with a reason, e.g. the netball shoe grips best because its soft rubber squashes into the floor’s high points (1). Change: the shoe; measure: spring-scale force as it starts to slide (static friction); keep the same: floor, slow level pull, and total mass, adding masses until each shoe reads the same on the kitchen scale (1, needs two controlled). Method: load each shoe to the same mass; pull level and read the force as it starts to slide; two trials each, averaged (2: 1 measurement, 1 repeats for all three). Supporting results: the predicted shoe has the biggest average (1). [5 marks]
Read the data: stopping on different roads
3. Dry asphalt (1): the shortest stop, 20 m, so the friction slowing the car was biggest (1). [2 marks]
4. Partly supported: more tyre grip stops the car sooner, 20 m on dry asphalt against 140 m on ice (1). But engine friction wears moving parts and wastes about 20% of the fuel’s energy, so oil is used to reduce it (1). More friction is better only where it is wanted (1). [3 marks]
Question 5
Marking guide: 1 static friction stops the walker’s shoe slipping; 1 a rough surface has bigger, more high points that interlock with the sole, so more grip; 1 a smooth surface has fewer, so less grip, risky on a steep slope; 1 rain fills the dips like a lubricant and keeps the high points apart, so a smooth wet path is very slippery; 1 recommends B with a reason; 1 names a cost (harder to clean, more wear on soles) and judges it worth paying. [6 marks]
Model answer: Static friction stops shoes slipping. A rough path has bigger high points that interlock with the sole, so it grips well; a smooth one has few, so feet slip more easily on a slope. Rainwater fills the dips like a lubricant, making a smooth path dangerous. I recommend B: it is harder to clean and wears soles faster, but safety matters more.
Common error: treating friction as always good or always bad.