Year 7 Science · Unit 3 · Lesson 8
Extension · Apply Worksheet
Learning Goals
Extension · optional
Pressure sits beyond the Stage 4 Forces dot points this unit is built on, and is met properly in later years. Nothing on this sheet is assessed as part of Year 7 Forces, so it is enrichment rather than required practice. Do it if you are interested, and skip it without losing any marks if you are not.
Compare two
The same 600 N person tries snowshoes and then stiletto heels. Complete both columns for each row.
| Feature | Snowshoes | Stiletto heels |
|---|---|---|
| Force on the ground | ||
| Contact area | ||
| Pressure on the ground | ||
| Best kind of ground to use them on |
1. Farm tractors in Australia have very wide tyres. Explain why, using pressure. What problem would narrow tyres cause for the crops?
Cause and effect: cutting butter
Pressing with the flat of a knife barely moves cold butter. Turning the same knife onto its edge cuts straight through. Complete the chain. Each box is caused by the one before it.
Read the data
Scenario: holes in a water bottle
Tom makes three small holes, one above the other, in a tall plastic bottle. He stands it on the edge of a bench and keeps topping it up so the water level stays the same. Water squirts out of each hole, and he measures how far from the bench each jet lands on the floor.
| Hole | Depth below the water surface (cm) | Distance the jet lands (cm) |
|---|---|---|
| Top | 5 | 42 |
| Middle | 15 | 70 |
| Bottom | 25 | 86 |
Source: sample class results.
2. Describe the pattern in Table 1. What does it tell you about how hard the water pushes out at different depths?
3. The wall of Warragamba Dam in NSW is much thicker at the base than at the top. Use Table 1 and the idea of pressure to explain why engineers build it this way.
Scenario: the rotten deck
A 70 kg person walks across an old backyard deck. Some planks are soft and slightly rotten. On one of these planks she stops, first standing on both feet in runners, then balancing on one foot.
4. Predict in which situation the rotten plank is more likely to crack, and justify your prediction. Then suggest one way she could cross the soft planks more safely, and explain why it works.
Compare two
1 per row, both columns right. Force: the same, 600 N, for both. Area: snowshoes large; stilettos tiny. Pressure: snowshoes low; stilettos very high. Ground: snowshoes on soft snow, sand or mud; stilettos only on hard floors that do not dent. [4 marks]
Question 1
Wide tyres spread the tractor's weight over a large area, so the pressure on the soil is low (1). Narrow tyres would give a high pressure that compacts (squashes) the soil and damages the crops (1). [2 marks]
Cause and effect
Box 1: the contact area with the butter becomes much smaller (1). Box 2: the same force is concentrated on that small area, so the pressure is much higher (1). [2 marks]
Question 2
The deeper the hole, the further the jet lands: 42, 70, 86 cm (1). Water pushes out harder the deeper you go: pressure increases with depth (1). [2 marks]
Question 3
Pressure in water increases with depth, because there is more water above (1). The bottom jet went furthest, so the push is greatest at the bottom (1). The wall must be strongest, so thickest, where the pressure is greatest (1). [3 marks]
Question 4
Marking guide: 1 prediction: more likely to crack on one foot; 1 the force (her weight, about 690 N) is the same both times; 1 the contact area halves on one foot; 1 so the pressure on the plank is higher (about double), and soft timber breaks when the pressure is too high; 1 a safer way: lay a wide board across the planks, or crawl or lie flat; 1 this spreads the same weight over a larger area, lowering the pressure. [6 marks]
Model answer: The plank is more likely to crack when she stands on one foot. Her weight is the same, but the contact area is halved, so the pressure on the rotten plank is about doubled. To cross safely she could lay a wide board across the planks: it spreads her weight over a larger area, so the pressure is lower.
Common error: saying she weighs more on one foot; the force is unchanged.