Year 7 Science · Unit 3 · Lesson 11
Challenge Worksheet
Learning Goals
True or false? Fix the false ones
Circle T or F. If a statement is false, rewrite it so it is true.
1. If the forces on a moving car are balanced, the car must be slowing down.
Correct it:
2. Gravitational, electrostatic and magnetic forces can all act across a gap.
Correct it:
3. A spring scale measures weight in newtons, not mass in kilograms.
Correct it:
4. A pulley system with a mechanical advantage above 1 lets you lift a load with less force and less rope.
Correct it:
5. A magnet can only attract another magnet, never push it away.
Correct it:
6. When friction stops a sliding book, the book’s movement energy is transferred to heat, not destroyed.
Correct it:
Design a mini-experiment
Investigation question: “Does the size of a paper parachute change how long it takes to fall 2 m?” You have sheets of paper, string, a 20 g lump of plasticine as the load, a metre ruler and a stopwatch. Plan a fair test.
What I will change (independent variable), and the values I will test
What I will measure (dependent variable), and how
What I will keep the same (at least 3 controlled variables)
My hypothesis (“If … is increased, then … will … because …”)
Why I will time three drops of each parachute and average them, rather than timing once
One limitation of this experiment, and how I could reduce it
Build the results table
Write the column headings, with units, for a table that records three trials and an average. Fill in the first column with the values you will test.
Read the data: another class’s parachutes
Scenario: the parachute drop
Another class ran this investigation with square canopies and the same 20 g load. Table 1 shows their times for a 2 m drop.
| Canopy side length (cm) | 10 | 20 | 30 | 40 |
|---|---|---|---|---|
| Drop 1 | 1.1 | 1.6 | 2.2 | 2.8 |
| Drop 2 | 1.0 | 1.5 | 2.1 | 2.7 |
| Drop 3 | 1.2 | 1.7 | 0.9 | 2.9 |
| Average | 1.1 | 1.6 | 1.7 | 2.8 |
Source: sample class results.
1. Name the two forces acting on a falling parachute, and say whether each is a contact or a non-contact force.
2. Identify the anomalous result, suggest one cause, and calculate a better average for that parachute.
3. Write a conclusion that describes the pattern in the data. Explain it using the forces on the parachute and the idea of balanced and unbalanced forces.
4. A friend says: “Forces are just a school topic. They have nothing to do with real life.” Write a response that challenges this claim. Use at least two ideas from this unit and one real Australian application.
True or false
1. False: balanced forces mean the car keeps a steady speed in a straight line. 2. True. 3. True. 4. False: you use less force but must pull more rope; no machine creates energy. 5. False: like poles repel. 6. True.
Design a mini-experiment
1 mark per row. IV: canopy size, e.g. squares of side 10, 20, 30, 40 cm (at least 3 sizes). DV: time to fall 2 m, measured with a stopwatch from release to landing. Controlled (need 3): same load, drop height, paper, string length; no drafts. Hypothesis, e.g. “If the canopy size is increased, then the fall time will increase, because a bigger canopy meets more air resistance.” Three drops: one bad drop cannot decide the result, and anomalies show up. Limitation, e.g. stopwatch reaction time: film the drop and time the video. [6 marks]
Build the results table
Headings: Canopy side (cm) | Trial 1 (s) | Trial 2 (s) | Trial 3 (s) | Average time (s) (1, units needed). Planned sizes in the first column (1). [2 marks]
Read the data: another class’s parachutes
1. Weight (gravitational force), non-contact (1). Air resistance, a contact force with the air (1). [2 marks]
2. Drop 3 of the 30 cm parachute, 0.9 s (1). Cause, e.g. the canopy did not open, or a timing slip (1). Better average: (2.2 + 2.1) ÷ 2 = 2.15 s, about 2.2 s (1). [3 marks]
3. The bigger the canopy, the longer the fall: 1.1 s at 10 cm to 2.8 s at 40 cm (1). Same weight each time, but a bigger canopy meets more air resistance pushing up (1). The forces are less unbalanced, so the parachute speeds up less and falls more slowly (1). [3 marks]
Question 4
Marking guide: 1 a clear response to the friend; 2 two unit ideas explained accurately (1 each); 2 each linked to a real use (1 each); 1 a real Australian application. [6 marks]
Model answer: That claim is wrong: forces explain everything that moves or stops. Friction is a contact force that opposes motion and turns movement energy into heat, which is how brakes stop a car. Balanced forces mean no change in motion: a car cruising steadily has its driving force balanced by friction and air resistance. Tower cranes in the Sydney CBD use block-and-tackle pulleys, a simple machine, so a small pull on the rope lifts heavy steel beams.
Common error: “balanced forces mean it is stopped”. They mean steady speed in a straight line, or at rest.