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### Chapter 3 Section 3

Energy and Work: Why Air Bags?

Physics 3.3: Energy and WorkHomework 1-10, Skip 4 and 7 Due Friday

- Learning Objective
- Students will be able to describe the relationship between stopping distance and force required to stop an object

- Success Criteria
- Model an automobile air bag
- Relate W=F·D to the eggsperiment (get it?)

- Do Now:
- QUIZ!

- Agenda
- Do Now
- Investigate 3.3
- Summary/Exit Ticket

Egg Lab

- Dropping Eggs!
- Throwing an egg in a blanket

Exit Ticket

- How did the indentation of the egg in the flour change as you increased the initial drop height?

Physics Talk 3.3

- Learning Objective
- Students will be able to explain the relationship between work and kinetic energy

- Success Criteria
- Students will be able to explain the relationship between stopping distance and force required to stop
- Students will be able to explain the relationship between velocity and braking distance
- Students will describe which features of a car help in the case of a collision

- Do Now:
- WDYS/WDYT p 277

- Agenda
- Do Now
- Physics Talk 3.3
- What do you think now?
- Summary

Physics Talk 3.3

- What is kinetic energy?
- What is the equation?

- The energy an object has because of its motion.

Kinetic Energy and the Eggs

- The two eggs were dropped from the same height. Which had more kinetic energy?

- Since both eggs had approximately the same mass and same relative velocity, they about the same kinetic energy.

Measuring Force Over a Distance

- How was the egg able to stop?
- How do you measure the distance in each egg drop?

- The surface had to apply a force over some distance.
- When the egg was dropped in flour, you can measure the indentation. With the hard surface, you cannot see the indentation.

Braking a Car

- How do auto brakes work?

- The braking system has to eliminate the kinetic energy of the vehicle by applying a large force over a large distance.

Work

- How do you calculate work?
- How does work relate to kinetic energy?

- W=F·D
- Work is the change in kinetic energy. We can write this as

What about the eggs?

- How does this apply to the eggs?

- When the egg could be stopped over a greater distance, there was less damage to it because less force had to be applied.

- Airbags! These allow you to have more distance to stop your body so you will be less likely to get hurt.

- If you are traveling three-times the speed, will you need three times the braking distance?

- False! You will need 9 times the stopping distance. This is because in the formula, the velocity is squared

- Why is it harder to stop on icy or wet roads? three times the braking distance?

- The slippery surface makes it more difficult to supply a sufficient force, so it is necessary to give yourself more distance.

- What else helps stop an automobile? three times the braking distance?

- The crumple zone of a car allows the car to have more distance to stop. This is why cars are designed to dent and are not made out of more sturdy material.

Sample Problem three times the braking distance?

- p286 and 287 Sample Problem

What do you think now? three times the braking distance?

Exit Ticket: How does and air bag protect you during an accident?

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