Ms. Parker's notes on Nov 17 - 18, 2011

1. Welcome Back!
Today, we will:
• Test your knowledge of accident statistics
• Perform a lab
• Take some notes
• Work some problems
Before the Tardy Bell:
• Get a clicker and your lab notebook
• Pick up the 15 question accident “quiz” and
begin working on the questions

2. Warm-Up
We will be taking the accident statistic “quiz” with
the clickers for your official score. The paper copy
is for your use.
Turn on your clickers in order to register them. Be
sure to register them with your last name and first
initial like this:
Parker_J

3. Seat Belts
In your lab book, start a new page. Title it
“Newton’s First Law of Motion and Seat Belts” Be
sure to make an entry in your table of contents as
well.

4. Seat Belts
Suppose you had to design a seat belt for a race
car that can go 300 km/h (about 200 mi/h). How
would it be different from one available on
passenger automobiles?
Make a heading in your lab book that will title
your thoughts and take five minutes to jot down
ideas for a safe race car seat belt.

5. Seat Belts
Share your ideas with your table partner. If you
think of other ideas, be sure to add them to your
lab book.
Now turn to your group of four and share your
ideas – make any additions or changes to your
ideas.

6. What Do You See?

7. Seat Belts
Turn in your textbook to page 266.
Read the Investigate carefully so that you
understand how you will conduct this lab.
Remember: each place in the text book that has a
pencil symbol is a place where you must write
in your lab notebook!

8. Review! Seat Belts
Newton’s First Law
An object at rest stays at rest
an object in motion stays in motion in a straight
line with constant speed
unless acted upon by a net, external force.

9. Law of Inertia
An object at rest stays at rest…
Think about the lab you just participated in and
write an example of an object staying at rest in
your notes.
“The part of the law of inertia that says an object
at rest stays at rest is shown by…”

10. Law of Inertia
“…an object in motion continues at the same speed in
a straight line…”
Think about the lab you just participated in and write
an example of an object staying at rest in your notes.
“The part of the law of inertia that says that an object
in motion continues at the same speed in a straight
line…”
BE CAREFUL! Gravity is an external force
acting on an object rolling down an incline!

11. Law of Inertia
“…unless acted upon by a net, external force.”
Think about the lab you just participated in and
write an example of a net external force.
“The part of the law of inertia that says a
net, external force will change the motion of an
object at rest OR an object in motion is shown

12. Take
Notes! 3 collisions in every accident
In every car accident, there are three collisions.
1. when the car strikes an object – such as another
car, a tree, a light-pole
2. as the car stops, the bodies in the vehicle continue
to move until they collide with the structure of the
car (seatbelt, steering column, air bags,
windshield)
3. the body stops, but the internal organs keep
moving until the body wall exerts a force on them
and brings them to rest

13. 3 collisions in every accident
While safety features such as seat belts and air
bags have dramatically reduced fatalities in
automobile accidents, it is still possible to suffer
serious or fatal consequences in a car crash
because of the third collision.

14. 3 collisions in every accident
In one recorded accident, a single vehicle with only
the driver inside failed to follow a turn and struck a
telephone pole. The seat belt and air bag prevented
any serious injuries apart from a few bruises, but the
driver died.
The autopsy showed that the driver’s aorta (the
largest blood vessel in the body – it leads out of the
heart to the body) tore in the collision of the heart
with the ribs and the man bled to dead internally.

15. What makes a seat belt safe?
In the second part of the lab, you investigated the
effect of different types of seat belts.
Keep in mind that the stopping force of each type
of seatbelt was the same because you released
the cart from the same height each time.
Yet, some seat belts did more damage to the clay
drivers than others. Why?

16. Take
Notes! Pressure
In the thin wire seat belt, all of the stopping force
was concentrated onto a small area. With a
broader strip of material, that same stopping force
was spread out over a larger area of contact.
Force that is spread out over a given area is called
pressure.

17. Pressure

18. Take
Notes! Pressure
In the picture, the pressure exerted by the feet of the
boy on the right is greater than the pressure exerted
by the feet by the boy on the left.
In our lab, the pressure of the wire as a seat belt was
greater than the pressure exerted by the ribbon.
It is the pressure, not force, that determines how
much damage a seat belt does to the body.

19. Take
Notes! Pressure Formula
Pressure = Force/area
where…
force is in Newtons
area is in square meters (m2)
and pressure is in N/m2 which is also called a
pascal.

20. Take
Notes! Sample Problem
Sample Problem
Two students have the same mass and apply a
constant force on the ground of 450 N while
standing in the snow.
Student X is wearing snowshoes that have a base
area of 2.0 m2. Student Y, without snow shoes, has
a base area of 0.1 m2.
Why does Student Y sink into the snow?

21. Take
Notes! Sample Problems
What is the pressure exerted when a force of 10 N
is applied to an object with the following areas?
a) 1.0 m2
b) 0.2 m2
c) 15 m2

22. A video
There is a famous demonstration of pressure using
a bed of nails.
Here is one version