--- title: "Newton's laws in the real world" subtitle: "Year 10 Physics · Newton's laws · Lesson 8" --- # Newton's laws in the real world **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** A crash test dummy hits an airbag and the force on it is far smaller than hitting the dashboard. Same crash, same dummy. What does a bag of air change? :::teacher-note **In one sentence:** Students apply Newton's laws to design features and techniques, explain how a First Nations spearthrower extends the force on a spear, and evaluate a vehicle safety feature using F = ma (AC9S10U05, node 8, an integration node). **Students should be able to say:** I can use two of Newton's laws to explain a real design or technique. I can explain how a spearthrower works in terms of force, time and acceleration. I can compare two safety designs with F = ma. **Width:** the unmarked parts are the floor every student does (Engage, Explore, Explain, Evaluate). Typical and Wider sections add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Engage :::keep-together **A crash dummy goes from 15 m/s to a stop either against a hard dashboard in 0.02 s or against an airbag in 0.2 s. Compared with the dashboard, the force from the airbag is:** ☐ About ten times smaller, because the acceleration is ten times smaller ☐ About ten times bigger ☐ Zero: the airbag absorbs it all ☐ The same, because the speed change is the same My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** About ten times smaller. The change in speed is the same, but taking ten times as long makes the acceleration ten times smaller, and F = ma. ::: ## Explain: design with the laws :::keep-together ### Which law **Match each feature to the law that explains it.** A. Second law: a smaller acceleration means a smaller force B. Second law: more mass, less acceleration C. First law: your body keeps moving unless a force acts D. Third law: action and reaction forces {{columns 60% 40%}} {{rows 1}} | Part | Letter | |---|---| | `Seat belt keeps you in your seat when the car stops` | | | `Airbag spreads the stop over more time` | | | `Rocket gases go down, rocket goes up` | | | `A heavier ball is harder to accelerate` | | ::: :::teacher-note **Answer:** Seat belt keeps you in your seat when the car stops → C. First law: your body keeps moving unless a force acts; Airbag spreads the stop over more time → A. Second law: a smaller acceleration means a smaller force; Rocket gases go down, rocket goes up → D. Third law: action and reaction forces; A heavier ball is harder to accelerate → B. Second law: more mass, less acceleration ::: :::keep-together ### The woomera **A spearthrower (woomera) is a long, light lever that holds the back of the spear. Why does it make a spear go faster than throwing by hand?** ☐ It makes the arm effectively longer, so the force acts on the spear for a longer time and distance, giving a bigger acceleration and a higher launch speed ☐ It removes air resistance ☐ It makes the spear heavier ::: :::teacher-note **Answer:** It makes the arm effectively longer, so the force acts on the spear for a longer time and distance, giving a bigger acceleration and a higher launch speed A longer push means more time to accelerate the spear, so it leaves at a higher speed. The same idea explains why a longer bow draw or a golf club swing launches faster. ::: :::keep-together ### Crumple zone **A 1000 kg car going 20 m/s is stopped by crumple zones in 0.5 s. What is its acceleration (m/s², size only)?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 40 a = 20 ÷ 0.5 = 40 m/s². ::: :::keep-together ### Crumple zone force **What average force is needed to stop it (N, size only)?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 40000 F = ma = 1000 × 40 = 40 000 N. ::: :::keep-together ### A better crumple zone **A better crumple zone stops the same car in 1.0 s instead of 0.5 s. What happens to the force on the people inside?** ☐ It doubles ☐ It stays the same ☐ It halves, so injuries are likely to be less serious ::: :::teacher-note **Answer:** It halves, so injuries are likely to be less serious Twice the stopping time gives half the acceleration and so half the force: 20 000 N. ::: :::typical ## Elaborate: sport :::keep-together **A footballer follows through when kicking a ball. In terms of force and time, why does the ball go further?** ☐ The foot stays in contact and pushes for longer, so the ball gains more speed ☐ The follow-through keeps pushing the ball after it has left the foot ☐ A follow-through reduces air resistance ::: :::teacher-note **Answer:** The foot stays in contact and pushes for longer, so the ball gains more speed A longer contact time means the force acts for longer, which gives the ball a higher launch speed. ::: ::: :::wider ## Elaborate: braking by itself :::keep-together **An automatic emergency braking system detects an obstacle 25 m ahead of a 1500 kg car at 20 m/s. What does it need to work out?** ☐ Whether the braking force the car can apply gives a stopping distance shorter than 25 m ☐ The colour of the obstacle ☐ Whether the car is under the speed limit ::: :::teacher-note **Answer:** Whether the braking force the car can apply gives a stopping distance shorter than 25 m It must compare the stopping distance from the car's speed and best possible braking force with the distance to the obstacle, quickly enough to act. ::: ::: ## Evaluate: look back Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - **First law** (inertia): seat belts and head restraints stop passengers carrying on moving when the car stops. - **Second law**: airbags and crumple zones lengthen the stopping time, so the acceleration and the **force** are smaller. A spearthrower extends the push, so the spear gets a higher launch speed. - **Third law**: a rocket pushes gas back and the gas pushes the rocket forward. - Numbers show how much a design helps: twice the stopping time halves the force. ## Exit ticket :::keep-together **A car stops from 20 m/s in 0.5 s with one crumple zone, and in 1.0 s with a better one. By what factor does the force on the occupants change?** {{lines-3}} ::: :::teacher-note **Answer:** It halves. Twice the time means half the acceleration, so half the force. :::