Year 7 Science · Unit 3 · Lesson 10
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Learning Goals
Compare the three classes of lever
Complete every empty cell. F = fulcrum, L = load, E = effort.
| Feature | Class 1 | Class 2 | Class 3 |
|---|---|---|---|
| Which part is in the middle? | |||
| Order of F, L and E along the bar | |||
| Mechanical advantage: greater than 1, less than 1, or either? | |||
| One example |
1. A bicycle is a compound machine. Name the simple machine in each part: (a) the pedal cranks turning the gear wheel; (b) the brake levers on the handlebars; (c) the sloping sides of the gear teeth.
Order the machines by effort
Each machine lifts the same 400 N load. Use effort = load ÷ MA to fill in the effort. Then rank the machines from 1 (smallest effort) to 5 (largest effort).
| Machine | MA | Effort (N) | Rank |
|---|---|---|---|
| Long Class 2 lifting bar | 5 | ||
| One movable pulley | 2 | ||
| Block and tackle on a tower crane | 8 | ||
| One fixed pulley on a beam | 1 | ||
| Block and tackle on a boat mast | 4 |
Plot the data: the ruler lever
Scenario: a metre-ruler lever
Ari rests a metre ruler on a pencil at its middle. A 4 N bag of washers hangs 10 cm from the pencil on one side. On the other side Ari pulls down with a spring scale, and records the effort needed to lift the bag at different distances from the pencil.
| Distance of effort from pencil (cm) | 10 | 20 | 30 | 40 | 50 |
|---|---|---|---|---|---|
| Effort (N) | 4.1 | 2.1 | 1.4 | 1.0 | 0.8 |
Source: sample class results.
2. Plot the data from Table 1 on the grid. Choose a scale for each axis, plot the five points and draw a smooth curve through them.
3. Describe the trend in your graph. Use it to predict the effort needed at 60 cm.
4. At 40 cm, Ari’s hand moves 4 times as far as the bag rises. Name the class of this lever. Then use the force–distance trade-off to explain why she can lift a 4 N bag with only 1 N.
5. At a boat workshop, Tahlia must lift a 600 N outboard motor 1 m off a boat. She can safely pull with at most 250 N. She has one fixed pulley, one movable pulley, and a block and tackle in which 4 rope segments hold the load. Recommend which set-up she should use, and justify your choice.
Compare the three classes of lever
Middle: Class 1 fulcrum, Class 2 load, Class 3 effort (1). Order: E, F, L (or L, F, E); F, L, E (or E, L, F); F, E, L (or L, E, F) (1). MA: either; always greater than 1; always less than 1 (1). Examples (1 for all three), e.g. seesaw; wheelbarrow; tweezers. [4 marks]
Question 1
(a) Wheel and axle (1). (b) Lever (1). (c) Inclined planes (1). [3 marks]
Order the machines by effort
Reading down the table as printed, the correct step numbers are 2, 4, 1, 5, 3. In order: tower crane 50 N, Class 2 bar 80 N, boat mast 100 N, movable pulley 200 N, fixed pulley 400 N. All five efforts (2; three or four = 1); ranking (1). [3 marks]
Plot the data: the ruler lever
2. Even scales that use most of the grid (1); five points plotted correctly (1); smooth curve through the points, not dot-to-dot straight lines (1). [3 marks]
3. The further the effort is from the fulcrum, the smaller the effort, falling by less each time (1). Prediction at 60 cm: about 0.7 N (accept 0.6 to 0.8 N) (1). [2 marks]
4. Class 1: the fulcrum (pencil) is between the effort and the load (1). The effort is a quarter of the load, 1 N instead of 4 N, so the MA is about 4 (1). In exchange her hand moves 4 times as far as the bag rises, so no energy is created: a smaller force acts over a longer distance (1). [3 marks]
Question 5
Marking guide: 1 recommendation: the block and tackle; 1 fixed pulley: MA 1, about 600 N, over 250 N; 1 movable pulley: MA about 2, about 300 N, still over 250 N; 1 block and tackle: MA about 4, so about 150 N, under the limit; 1 she must pull about 4 m of rope to raise the motor 1 m; 1 reason: a smaller force acts over a longer distance, as no machine creates energy (accept: friction adds a little to the 150 N, still under 250 N). [6 marks]
Model answer: Tahlia should use the block and tackle. A fixed pulley only changes direction, so she would still need about 600 N. A movable pulley halves it to about 300 N, still over her limit. With 4 rope segments holding the motor, each carries a quarter of the load, so she needs only about 150 N, a little more once friction is included. The price is distance: she pulls about 4 m of rope to raise the motor 1 m; the machine trades force for distance and never creates energy.
Common error: effort = load × MA instead of load ÷ MA.