Impact forces

1. Impact forces P2.3.6

2. Starter • In any collision between vehicles there are three collisions: • Vehicle to vehicle • Passenger to vehicle (hopefully via a seat belt!) • Internal organs to passenger’s body • Car accidents can cause serious harm. • List all of the safety features you can that are put into cars to help reduce injury to passengers.

3. Learning outcomes • By the end of this lesson you will: • KNOW that a resultant force will change the momentum of an object. (D/E) • DESCRIBE the factors that affect the size of a force in an impact. (C/D) • BE ABLE TO calculate the force produced in a collision using: (A/B)

4. To begin with… • Two vehicles are travelling toward each other. They are designated (labelled) A and B. • Find the kinetic energy of A and B. • Find the momentum of A and B. • Find the total momentum. Remember:

5. To begin with…

6. To begin with… • The vehicles collide and stick together. • What direction do they continue to travel in? • Use the calculation you have just completed to justify your answer. • How fast do the two vehicles travel after the collision?

7. To begin with… • The vehicles collide and stick together. • Momentum is conserved! The momentum before the collision must be the same as the the momentum after the collision!

8. Let’s play catch! • I’ll need two volunteers for this! • It would help if these people can catch! • What do you notice about how the catching technique changes for the egg? • Why might this be?

9. Why the change in technique? • Instinctively our volunteers know that the egg will crack with too much force – and they don’t want that! • They try to be ‘more gentle’ in the way they catch the egg. • To do this they bring the egg to a stop over a longer period of time using less force. • For mathematicians: Final momentum Starting momentum

10. A new definition of force! LO1 – Resultant force will change momentum of an object • So we now know that force has another definition – one that is useful to us here! • What does Δ mean? • What happens to the force as the time gets larger? • What happens to the force as the time gets smaller? • Can you link this to crumple zones in cars?

11. It’s just like catching an egg! • Changing the amount of time it takes to increase or decrease the speed of an object changes the amount of force required. • Crumple zones and roll bars increase the impact time and so reduce the force. • What other factors affect the size of an impact force? • Remember:

12. It’s just like catching an egg! LO2 – When vehicles collide, the force of impact depends on mass, change in velocity and duration of the impact. • What other factors affect the size of an impact force? The change in momentum is the change in velocity multiplied by the mass Change in velocity time (duration of impact)

13. So, how does this work? • Copy and complete this table:

14. So, how does this work? LO3 – Be able to calculate impact force. • And here are the answers!

15. Some questions • Why is rubber matting under a child’s swing a good idea? • In a car crash, why does a seat belt reduce the impact force on the wearer? • An 800kg car travelling at 30m/s is stopped safely when the brakes are applied. What acceleration and braking force is required to stop it in a) 6.0s? b) 30s? • If the vehicle in the previous question had been stopped in a collision lasting less than a second, explain why the force on it would have been much greater. • A 2000kg van moving at a velocity of 12m/s crashes into the back of a stationary truck of mass 10,000kg. Immediately after the impact the two vehicles move together. a) Show that the velocity of the can and the truck immediately after the impact was 2m/s. b) The impact lasted for 0.3 seconds. Calculate i the deceleration of the van ii force of impact on the van.

16. Learning outcomes • Can you show that you: • KNOW that a resultant force will change the momentum of an object. (D/E) • can DESCRIBE the factors that affect the size of a force in an impact. (C/D) are ABLE TO calculate the force produced in a collision using: (A/B)

17. How can we apply what we have learnt to our lives? • Car manufacturers need to sell cars to make money. • Safety features cost money and make cars more expensive. • What do you think happens if cars are very safe but very expensive?

18. How can we apply what we have learnt to our lives? • You have now learnt about momentum, the change in momentum and how this relates to impact forces. • For your homework you are going to design a campaign to encourage people top use the available safety features in their vehicles and to look for good safety features when buying cars.

19. How can we apply what we have learnt to our lives? • You will need to include: • Seat belts. • Air bags. • Child car seats. • Side impact bars. • Remember! You need to include the science behind these safety features. • We are going to watch a series of videos to help you.

20. How can we apply what we have learnt to our lives? • Video 1 • Video 2 • Video 3 • Airbags • Crumple Zones Student Video • Child Seat • Child Seat

21. Your campaign • Your information may be presented however you like. • You could: • Produce a short video. • Produce a webpage. • Design a poster. • Write an article. • Design a leaflet. • If you have any other ideas then please let me know!