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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.

How does this apply to a car crash?

- 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?

- 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?

- 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

- p286 and 287 Sample Problem

What do you think now?

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

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