# Chapter 3 Section 3 - PowerPoint PPT Presentation

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Chapter 3 Section 3. Energy and Work: Why Air Bags?. Physics 3.3: Energy and Work Homework 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

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Chapter 3 Section 3

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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

### Example

• 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?