Year 7 Science · Unit 3 · Lesson 3
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Learning Goals
True or false? Fix the false ones
Circle T or F. If a statement is false, correct it on the line.
1. Friction acts in the opposite direction to the motion, or to the motion something is trying to make.
Correct it:
2. Rolling friction is much smaller than sliding friction.
Correct it:
3. If you push gently on a heavy box and it does not move, there is no friction acting on it.
Correct it:
4. Pressing two surfaces together harder increases the friction between them.
Correct it:
5. Oil reduces friction by making the surfaces rougher.
Correct it:
6. Once a box is sliding, pushing it harder makes the sliding friction on it much bigger.
Correct it:
Order the steps: starting a heavy box moving
Number the events from 1 (first) to 5 (last).
| Step | Event |
|---|---|
| Your push becomes bigger than that biggest grip, the high points shear, and the box breaks away into a slide. | |
| The high points of the box and floor grip back with static friction equal to your push, so the box stays still. | |
| Sliding friction now acts. It is smaller than the biggest static grip, so a smaller push keeps the box moving. | |
| You push harder, and static friction grows to match you, until it reaches the most grip the high points can manage. | |
| You push gently on a heavy box resting on the floor. |
Increase or reduce?
For each problem, decide whether friction should be increased or reduced. Choose a method from the lesson and explain how it works.
| Problem | Increase or reduce? | Method, and how it works |
|---|---|---|
| 1. A door hinge squeaks and sticks | ||
| 2. A skateboarder’s shoes slip off the deck | ||
| 3. A worker drags heavy boxes across a warehouse floor | ||
| 4. A cyclist is slowed by a strong headwind |
Read the data: starting and keeping a block moving
Scenario
A class pulled a 1 kg wooden block with a spring scale. They recorded the force at the moment the block started to move, then the force needed to keep it moving steadily. Last, they rested the block on three round pencils as rollers.
| Set-up | Force to start it moving (N) | Force to keep it moving (N) |
|---|---|---|
| Block on the bench top | 4.2 | 3.0 |
| Block on sandpaper | 7.6 | 6.1 |
| Block on pencil rollers on the bench top | 0.5 | 0.3 |
Source: sample class results.
1. Describe the pattern when you compare the two force columns for each set-up. Which type of friction does each column measure for the block on the bench top?
2. Use the idea of interlocking high points to explain (a) why the sandpaper needed the biggest forces, and (b) why the rollers needed the smallest forces.
3. At the Bathurst 1000, race teams choose between soft and hard tyre rubber. Soft tyres grip the road better but wear out faster. Explain why softness causes both effects, and why a team does not simply use the softest tyres all race.
True or false
1. True. 2. True. 3. False: static friction matches the push. 4. True. 5. False: oil fills the dips and keeps the high points apart. 6. False: sliding friction stays about the same; the extra push speeds the box up. 1 mark per two correct. [3 marks]
Order the steps: starting a heavy box moving
Reading down the table as printed, the correct step numbers are 4, 2, 5, 3, 1. In order: gentle push; static friction matches it; push harder, static friction grows to its limit; box breaks away; smaller sliding friction.
Increase or reduce?
1 mark per row, both cells correct. 1: reduce; oil (lubricant); fills the dips and keeps the surfaces apart. 2: increase; grip tape; bigger, sharper high points dig in. 3: reduce; a trolley with wheels; swaps sliding for much smaller rolling friction. 4: reduce; streamline (crouch low); less air to push aside, less fluid friction. [4 marks]
Read the data: starting and keeping a block moving
1. In every set-up the force to start is bigger than the force to keep it moving (e.g. 4.2 N vs 3.0 N on the bench) (1). Start column: static friction; keep-moving column: sliding friction (1). [2 marks]
2. (a) Sandpaper is rougher, with bigger and more high points to interlock with the block, so more must be sheared past (1). (b) On rollers the block rolls instead of sliding (1); each pencil touches at a small patch that is set down and lifted, not dragged, so the high points never shear past each other (1). [3 marks]
Question 3
Marking guide: 1 soft rubber squashes down into the road’s high points and moulds around them; 1 so more rubber is in real contact with the road, which gives more grip; 1 the same softness lets more rubber be torn away each lap, so the tyre wears faster; 1 hard rubber lasts longer but grips less; 1 a soft tyre used all race would wear out, losing its grip or needing to be replaced; 1 so the team balances grip against wear for each part of the race. [6 marks]
Model answer: Soft rubber squashes into the road’s high points, so more rubber really touches the road and grips harder. The same softness lets more rubber tear away each lap, so it wears first. Hard rubber lasts longer but grips less. A soft tyre used all race would wear out and lose grip, so teams balance grip against wear.
Common error: thinking friction on a still object is zero, or that pushing a sliding box harder increases the friction on it.