A trade-off answer states what a material gains and what it gives up, then recommends it for a named use. It does not announce a winner for every situation.
This lesson follows choosing a material from a property table. It belongs to the set on materials and technology through practical evidence.
What is the sentence frame?
Use one sentence for the gain and one for the cost.
- “Material X is better for [use] because [property with a figure].”
- “However, it gives up [other property with a figure].”
If a third sentence is allowed, add what would change the recommendation.
Worked example: two fictional bottles
An original table compares two bottles that hold the same volume of water.
| Bottle | Mass | Cost per bottle | After 100 scratch-test strokes |
|---|---|---|---|
| A (hard-coated alloy) | 120 g | RM25 | No visible marks |
| B (light polymer) | 45 g | RM8 | Clear scratches |
Step 1: compare the figures. A has a mass of 120 g and B has 45 g, so B is 120 − 45 = 75 g lighter. B costs RM8 and A costs RM25, so B is RM25 − RM8 = RM17 cheaper.
Step 2: scale to the situation. A class of 30 students carrying B instead of A would save 30 × 75 g = 2 250 g = 2.25 kg of total mass and 30 × RM17 = RM510 in cost.
Step 3: state the trade-off. B is better for carrying to school because it is 75 g lighter and RM17 cheaper. However, it gives up scratch resistance, because B showed clear scratches after 100 strokes while A showed none.
Step 4: tie it to a use. For a student who carries the bottle daily, B suits. For a bottle used on a rough worksite, A’s scratch resistance may matter more, so the recommendation would change.
Assumption: both bottles hold the same volume and the scratch test applied the same force.
The mistake that costs marks
The slip is to write a single verdict: “Bottle B is the best.” It sounds sure, but it ignores the scratch result and does not name a use.
| Wrong | Right |
|---|---|
| “B is best” | “B is better for carrying to school” |
| “A is expensive” | “A costs RM17 more, in exchange for scratch resistance” |
| “B is bad because it scratches” | “B gives up scratch resistance, which matters less for a school bag” |
The repair is to add “for” and a use to every verdict, and to attach a figure to every property.
Check yourself
An original table compares two casings for a small drone. Casing M has a mass of 30 g and cracks if dropped from 1 m. Casing N has a mass of 50 g and survives a drop from 3 m.
The drone’s flight time falls by 2 minutes for every extra 10 g carried. Explain the trade-off and recommend one for a beginner who will crash the drone.
Answer
N is 50 − 30 = 20 g heavier, so the flight time falls by 2 × (20 ÷ 10) = 4 minutes compared with M. N survives a drop from 3 m, while M cracks from 1 m.
Trade-off: M gives longer flight, but it gives up crash resistance. N gives up 4 minutes of flight but survives much higher drops.
Recommend N for a beginner who will crash the drone, because breaking the casing would cost more than 4 minutes of flight. For a flight-time competition with careful pilots, M could be the better choice.
What to study next
Continue with connecting a material model to a measured property. The claim evidence revision desk helps you rewrite an answer until the gain, the cost and the use all appear.
For help with comparison writing, see online one-to-one Science tuition.