Why Use Seat Belts?

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

Objective:
     To show some reasons for wearing seat belts.

Materials:
   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

Strategies:

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 
barricade. 

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 
     collision.
 
 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!