Year 7 Science · Unit 3 · Lesson 5
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Learning Goals
Scenario
Two vehicles are both travelling at 60 km/h on a suburban road in Sydney when they apply their brakes at exactly the same moment. Vehicle 1 is a fully loaded semi-trailer with a total mass of 42,000 kg. Vehicle 2 is a standard bicycle with a rider — total mass of 90 kg. Both vehicles apply the same braking force relative to their size.
(a) Predict: Which vehicle stops in a shorter distance? Write your prediction clearly.
(b) Justify: Use the scientific ideas of inertia and Newton's First Law to explain your prediction.
Real-world context
In 2023, the Australasian New Car Assessment Programme (ANCAP) conducted crash tests on new vehicles at its facility in Victoria. During a 64 km/h frontal crash test, high-speed cameras showed that when the car stopped in about 0.1 seconds, the crash test dummy's body continued to move forward — exactly as a real person's body would. The dummy's chest was stopped from hitting the steering wheel only by the seatbelt, which stretched slightly to extend the stopping time.
(a) Using Newton's First Law, explain why the dummy's body continued to move forward after the car stopped.
(b) The seatbelt "stretched slightly to extend the stopping time." Why does this make it safer than a completely rigid belt that stops the body instantly?
1. A passenger on a bus is standing and holding a rail. The bus brakes suddenly. Using Newton's First Law, explain what happens to the passenger's body and why.
2. Victoria introduced the world's first compulsory seatbelt law in 1970. The law was designed around Newton's First Law. In your own words, explain the scientific reason why seatbelts are compulsory.
Wrap Up
In one sentence, what was the main idea of this lesson?