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
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?

The energy an object
has because of its
motion.
What is the equation?
1 2
K E  mv
2
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
W  KE
BRAKING A CAR
EXAMPLE
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
1 2
KE  mv
2

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?