Forces That Transfer Energy Making Crash Barriers

Crumple Zones

Collapsed Crumple Zone

Good Crumple Zone

Focus Question: What barrier design will stop the car in the shortest distance?

Investigating How Forces Transfer EnergyPart A: Creating a Barrier

Your task is to create a stopping barrier out of dominoes that will stop the car in the shortest distance possible.

Your Mission…1. You will answer the pre-lab questions (1-4) BEFORE you start the lab.2. Practice three or four times beforeyou record your trials.3. You will stack the dominos at the 60cmmark on the ramp.4. Draw your design in your table.5. Record the distance in cm from the 60cm

mark to the first domino.6. If your dominos touch the end wall, your trial

doesn't count!7. You can not have any dominos touching the

black wall at the end of ramp.

Design your Drawing

Draw your design as if you were looking down at it from above.Place numbers in the blocks if you stacked more than one on each other.

3

23

13

21

Data TableTrial #

# of blocks used

Draw a basic barrier design

Stopping distance

(cm)1

2

3

4

5

Pre-Investigation Questions

• Question #1: What form of energy is present when the car is sitting at the top of the ramp? How do you know this?

• Question #2: What will happen to the energy of the car as it moves down the ramp? What evidence could you collect to justify your answer?

• Question #3: When the car strikes the barrier what will happen to the energy of the car? How do you know this?

• Question #4: Let’s assume we release the car from rest at the top of your ramp. What can you do to be sure that the car strikes your barrier with the same KE in each trial? Explain.

Record your results carefully and be prepared to report to the class the design of your barrier that stopped the car in the shortest distance by exerting the largest stopping force and the answers to the questions asked below.

Question #5: What forces are causing the car to stop?

Question #6: Why is the stopping distance shorter for some arrangements of blocks than for other arrangements?

Conduct your Investigation

A child is playing with a toy dart gun. The dart is inserted into the barrel and it compresses a spring. When the trigger is pulled, the spring forces the dart out of the gun. Carefully read each of the following statements about the scenario and write the statement that you agree with.1.The spring inside the dart gun is elastic.2.The energy stored in the compressed spring is the same as the weight of the dart that caused the spring to compress.3. If the spring is compressed twice as far, the dart will go twice as far.4. If the spring is compressed twice as far, the dart will go twice as fast.5. The energy stored in the compressed spring is a type of potential energy.

Elastic Potential EnergyElasticity is the ability of an object to be

deformed from its original shape by a force and return to its original shape. (When an object’s shape is changed it can be returned back to it’s original shape)What does deformed mean?A change in the shape or size of an object due to an applied force, such as the stretching of a spring or rubber band. Can you thin k of any other examples of Elastic PE?

Bow and Arrow, Sling Shot, Balloons

Elastic Potential Energy

Elastic Potential energy can be determined from the objects stretch squared and the constant that reflects the elasticity of the material. So what’s this mean in English?

Elastic EPE= 1/2kx2

Elastic Potential Energy

Elastic PE= 1/2kx2

K=spring constant N/m

X= amount stretched or compression

Ex: (stretched spring, pulled rubber band)

Elastic PE is measured in Joules N/m x m(Kg x m/s2 x m)

Elastic Potential Energy Problem

Remember… Elastic PE= 1/2kx2

The elastic force constant stored in a drawn bow is 100 N/m. The bow is drawn to pull the arrow back a distance of 0.5 meters. Calculate the elestic potential energy stored in the drawn bow.

In DUFAS

K= 100 N/m 2 50

N/mX=(.5m)2

.25m

50 N/m x .25m EPE =12.5 J

Elastic Potential Energy Problem

The elastic force constant of a spring in a toy is 550 N/m.  If the spring is compressed .12 m, compute the elestic potential energy stored in the spring.

In DUFAS

K= 550 N/m 2 275

N/mX=(.12m)2

.0144m

275 N/m x .0144m

EPE =3.96 J

SAFER Crash Barriers

An excellent application of these concepts is the “soft walls” used by major racing facilities across the nation (Dover International Speedway being one of these). The new SAFER (Steel And Foam Energy Reduction) barriers have revolutionized the sport of automobile racing and made it much safer for both the drivers and the fans.

So how do SAFER barriers absorb energy? The barriers move upon impact so that the KE of the car is transferred to a very large area of the wall (a large portion of the wall flexes upon impact). The key idea is that no one portion of the wall receives a large amount of the car’s KE. The KE of the flexing soft wall is then transferred to the outer permanent wall and support structure. The materials that make up the wall are not elastic.

Imagine what the collision would be like if the wall was elastic! Still other portions of the car’s initial KE are transformed into heat energy and sound energy.