Year 7 Science · Unit 3 · Lesson 5
Extension · Apply 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.
Fill the gap
Choose the correct word from the word bank. Each word is used at most once. Two words are not needed: cross them out.
Newton's First Law says that an object stays at , or keeps moving at a constant , unless an force acts on it. The tendency of an object to resist any change in its motion is called . It depends only on its , not on its size or its .
Sort it
What is inertia doing in each event? Write each event in the correct box. The boxes do not hold the same number of items.
Inertia keeps it still
Inertia keeps it moving
Scenario: same speed, same brakes
A loaded semi-trailer (total mass 42 000 kg) and a bicycle with its rider (total mass 90 kg) are both travelling at 60 km/h. They brake at the same moment, and the same size of braking force acts on each.
1. (a) Predict which one stops in the shorter distance. (b) Justify your prediction using inertia and Newton's First Law.
Cause and effect: a bus stops suddenly
Fill the two empty boxes. Each box is caused by the one before it, and row 2 carries on from the end of row 1.
Read the data
Scenario: two crash-test dummies
A crash-test lab drives a car into a solid barrier at 64 km/h. Two dummies ride inside: one wears a seatbelt and one does not. High-speed cameras measure the speed of the car and of each dummy’s chest.
| Time after impact (s) | Car | Belted dummy | Unbelted dummy |
|---|---|---|---|
| 0.00 | 64 | 64 | 64 |
| 0.03 | 35 | 45 | 64 |
| 0.06 | 8 | 20 | 64 |
| 0.09 | 0 | 2 | 0 (has hit the steering wheel) |
Source: made-up values based on a 64 km/h crash test.
2. From 0.00 s to 0.06 s the car slowed from 64 to 8 km/h, but the unbelted dummy’s speed did not change at all. Use Newton's First Law to explain why.
3. Compare how the belted and the unbelted dummies were stopped. Use Table 1 to explain why the seatbelt makes the crash safer.
4. In a school zone the speed limit drops to 40 km/h. Leo says: “At 40 km/h my inertia is much smaller, so I don’t need my seatbelt in a school zone.” Evaluate Leo’s claim using Newton's First Law and inertia.
Fill the gap
rest; velocity; unbalanced; inertia; mass; speed. Not used: gravity, balanced. Accept blanks 2 and 6 swapped. Six correct = 3; four or five = 2; two or three = 1. [3 marks]
Sort it
Keeps it still: dishes and tablecloth; bus passenger tips backward as the bus pulls away; loaded wheelbarrow hard to get moving. Keeps it moving: skateboarder flies forward; cricket ball keeps travelling; space probe coasts on; loaded truck needs a long distance to stop.
Question 1
(a) The bicycle (1). (b) The semi-trailer has hundreds of times more mass, so far more inertia (1). By the First Law each keeps moving until an unbalanced force stops it; the same braking force changes the semi-trailer’s motion only slowly, so it travels much further (1). [3 marks]
Cause and effect: a bus stops suddenly
Box 1: the braking force acts on the bus, so the bus slows down (1). Box 2: nothing pushes the passenger back yet, so by inertia their body keeps moving forward at the old speed (1). [2 marks]
Read the data: Question 2
The barrier acted on the car, but no unbalanced (backward) force acted on the dummy (1), so by the First Law it kept moving at 64 km/h (1). [2 marks]
Question 3
Belted: slowed with the car, 64 to 2 km/h over 0.09 s (1), as the belt applied an unbalanced force at once (1). Unbelted: stayed at 64 km/h, then stopped suddenly on the steering wheel (1). [3 marks]
Question 4
Marking guide: 1 judgement (Leo is wrong); 1 inertia depends on mass only; 1 not on speed, so Leo’s inertia is the same at 40 km/h; 1 in a sudden stop his body keeps moving at 40 km/h (First Law); 1 only an unbalanced force (the belt) stops him with the car; 1 the lower speed only means less motion to be stopped, not no danger. [6 marks]
Model answer: Leo is wrong: inertia depends only on mass, and he is no lighter at 40 km/h. In a sudden stop his body keeps moving at 40 km/h until a force stops it; without a belt, that is the dashboard. Less speed means less motion to stop, not safety.
Common error: saying a force “throws” the passenger forward; they simply keep moving.