Why Use Seat Belts?

Larry Brandon Thornwood High School
17000 South Park Avenue
South Holland IL 60473
(708) 339-7800

To show some reasons for wearing seat belts.

Per Group:
Board (for ramp - length about .75 m to 1.0 m)
Dynamics cart
Books (for propping up one end of board + 1 for barrier)
Clamp (to fasten barrier down to table top)
Plasticene (or modeling clay)
Meter stick or ruler

For Teacher Demonstration:
Barbie doll or similar type doll
A seat and dash assembly


Number 1: Teacher Activity
As a demonstration, put a Barbie doll on a block seat on a dynamics cart.
Place the cart at the top of a ramp which is set with the top 30 cm above the
table top. Clamp a book down as a barricade about 50 cm from the bottom of the
ramp. Release the cart and observe where the doll ends up after hitting the

Number 2: Student Activity
Mark the ramp into 20 cm intervals. Raise one end of the ramp about 30 cm.
Position an obstacle such as a book about 30 cm from the bottom of the ramp.
Hold the obstacle stationary.
Make a plasticene (modeling clay) cube "passenger" with sides about 2 cm
long. Place the passenger on the front of a dynamics cart. Place the front of
the dynamics cart at the 20 cm mark on the ramp. (It may make results more
graphic if students shape the plasticene into a roughly human shape.)
Release the cart. Observe the motion of the passenger during and after the
collision. Measure the distance the passenger moves from the collision point to
where it stops. Repeat this step several times and average the distance.
Release the cart from several different distances up the incline to vary
the speed. Observe the motion of the passenger and measure the distance as in
the previous paragraph. Repeat this procedure several times for each measured
20 cm mark to average the distances.

Possible Extensions:
By spending some money at your local Venture or similar store, you can
obtain some "Crash Dummies" and a "Crash Dummy Car." They claim to have a
"Crash Dummy Cycle" as well. The car has a facility built in for attaching seat
belts. It also gives a very satisfactory crash in that the roof and windshield
and front wheels go flying off. If the dummies hit hard enough, they may also
fly apart. Students should also get a thrill from this.
Another extension would be to use ticker tape timers and attach ticker
tape to each dummy as well as the car. This would provide a difference in time
of movement as well as distance differences.

Performance Assessment:

(The following questions should be made available. If students have
trouble with the questions, the equipment should still be available. They
should obtain their equipment again and do what is needed in order to answer
the questions.)

1. Describe the motion of the passenger during and after the front-end

2. How did the speed just before the collision change as the cart was released
from further up the ramp?

3. How did the distance the passenger rolled after the collision change as the
cart was released from further up the ramp?

4. Describe the motion of an unbelted passenger in a car which collides with a
stationary obstacle.

5. Draw a diagram of the forces acting on the cart and on the passenger on the
flat before and during collision.

6. Seatbelts prevent a passenger from being thrown from the car. Why is it
usually more dangerous to be thrown from the car than to remain in it?

7. Newton's First Law says (in essence), it takes an unbalanced force to
change velocity. Explain how this applies to the motion of the passenger
during the collision.

Multi-Cultural Implications:

The implications of this laboratory experiment tend to be related to age
rather than culture. My observation tends to the idea that mature individuals
use seatbelts more than teenagers do. However, European countries do not have
speed limits on their highways. Therefore, the Europeans do wear seat belts
more than Americans do!
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