--- marp: true theme: default html: true --- # Newton's laws in the real world Year 10 Physics · Newton's laws --- ## Today **We are learning to** newton's laws explain why safety features and techniques work. Seat belts and head restraints deal with inertia (first law); airbags and crumple zones lengthen the stopping time to cut the force (F = ma); rockets push gases back and are pushed forward (third law); and a spearthrower (woomera) extends the thrower's arm so the force acts for longer and over a greater distance, so the spear leaves faster. - 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. --- ## Why it matters > 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? --- 💬 Discussion ## Engage: predict **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 --- ✅ Check ## What happened? * 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. --- ✅ Check ## Which law **Match each feature to the law that explains it.** | | | |---|---| | `Seat belt keeps you in your seat when the car stops` | A. Second law: a smaller acceleration means a smaller force | | `Airbag spreads the stop over more time` | B. Second law: more mass, less acceleration | | `Rocket gases go down, rocket goes up` | C. First law: your body keeps moving unless a force acts | | `A heavier ball is harder to accelerate` | D. Third law: action and reaction forces | * 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 --- ✅ Check ## 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 * 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 --- ✅ Check ## 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: 40 --- ✅ Check ## Crumple zone force **What average force is needed to stop it (N, size only)?** * Answer: 40000 --- ✅ Check ## 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 * Answer: It halves, so injuries are likely to be less serious --- ✅ Check ## Elaborate: sport **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 * Answer: The foot stays in contact and pushes for longer, so the ball gains more speed --- ✅ Check ## Elaborate: braking by itself **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 * Answer: Whether the braking force the car can apply gives a stopping distance shorter than 25 m --- ## 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. --- ✅ Check ## Exit ticket **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?** * It halves. Twice the time means half the acceleration, so half the force.