Year 7 Science · Unit 3 · Lesson 5
Extension · Challenge Worksheet
Learning Goals
Extension · optional
Newton's First Law and inertia sit in Stage 5 (SC5-WAM-01), beyond the Stage 4 Forces dot points this unit is built on. 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.
1. Explain the difference between Newton's First Law and inertia.
Find the mistakes
Noor wrote this summary
“When a car brakes hard, inertia pushes the passengers forward. A beach ball has more inertia than a bowling ball, because it is bigger. And a space probe that turns off its engine far from any star will slowly run out of speed and stop.”
2. Noor’s summary contains three errors. Quote each one and give the correct science.
3. A space probe is coasting at 15 km/s far from any planet or star, with no forces acting on it. Mission control fires a thruster that pushes it forward for 10 minutes, then switches the thruster off. Describe the probe’s motion (a) before the thruster fires, (b) while it fires and (c) after it switches off. Explain each using Newton's First Law.
Plan a short investigation
Question to test: “Does the mass of a trolley affect how far it rolls after the same push from a stretched rubber band?”
Hypothesis, with a reason that uses inertia
Variables
Method (3 steps)
Read the data
Results from another class
Another class ran this test. They pulled the rubber band back the same distance every time, on the same smooth floor.
| Mass of trolley (kg) | Run 1 | Run 2 | Run 3 | Mean |
|---|---|---|---|---|
| 0.50 | 122 | 118 | 120 | 120 |
| 0.75 | 79 | 83 | 81 | |
| 1.00 | 61 | 36 | 59 | |
| 1.25 | 47 | 49 | 48 | 48 |
Source: sample class results.
4. Work out the two missing means. Leave out any run that does not fit the others, and say which one you left out.
5. A student’s hypothesis was: “Heavier trolleys roll further, because their inertia makes them harder to stop.” Describe the pattern in Table 1 and use inertia to judge this hypothesis.
6. Evaluate this claim: “Newton's First Law can’t be right, because on Earth every moving thing eventually stops.” Use one example on Earth and one in space.
Question 1
The law is the rule: rest or constant velocity unless an unbalanced force acts (1). Inertia is the property that resists change in motion, set by mass (1). [2 marks]
Find the mistakes: Question 2
1 each: “inertia pushes”: not a force; they keep moving as nothing stops them. “because it is bigger”: inertia depends on mass, so the bowling ball has more. “run out of speed”: with no force it keeps its speed. [3 marks]
Question 3
(a) Steady 15 km/s, straight line: no force (1). (b) Speeds up: unbalanced force (1). (c) Keeps its new, higher speed: no force (1). [3 marks]
Plan a short investigation
Hypothesis with an inertia reason, e.g. heavier rolls less far, as the same push changes its motion less (1). Change: mass; measure: distance rolled (1). Three of: same band and pull-back, floor, trolley, start line (1). Method: add masses in steps and launch (1); measure to where it stops (1); repeat 3 times per mass and average (1). [6 marks]
Read the data: Question 4
0.75 kg: (79 + 83 + 81) ÷ 3 = 81 cm (1). 1.00 kg: 36 is left out; (61 + 59) ÷ 2 = 60 cm (1). [2 marks]
Question 5
More mass, shorter distance: 120 cm at 0.50 kg to 48 cm at 1.25 kg (1). The same push changes a heavier trolley’s motion less, so it starts slower: inertia resists starting too (1). Not supported (1). [3 marks]
Question 6
Marking guide: 1 judgement (wrong); 1 friction and air resistance act on Earth; 1 as unbalanced forces they slow things (the law); 1 Earth example (e.g. smoother floor, ball rolls further); 1 space example (probe coasts on); 1 surprising because on Earth these forces are always present. [6 marks]
Model answer: Wrong. On Earth, friction and air resistance are unbalanced forces that slow things. A ball rolls further on a smoother floor, with less friction. A probe far from any star, with no forces, coasts on. The law surprised people because on Earth these forces never go away.
Common error: predicting heavier trolleys roll further.