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NOVA scienceNOW: Brain Trauma

Classroom Activity

Activity Summary
Students investigate how model bicycle helmets protect the brain from injuries sustained during a crash.

Learning Objectives
Students will be able to:

  • explain how a sudden change in the motion of the brain inside the skull during a crash can cause brain injuries.

  • describe how a bicycle helmet prevents serious head injuries.

  • understand the relationship between force and acceleration (Newton's 2nd Law).

  • explain how increasing the time of deceleration during a crash decreases the force of impact.

Suggested Time
One class period

Materials for each team
  • Protecting your Brain: The Egghead Bicycle Helmet student handout (PDF)
  • 3 eggs per pair of students (or per student group)
  • stapler
  • tape
  • a variety of cushioning materials per group, such as hard and soft foam, (about 1 sq. yd. per class–purchase at any craft store), bubble wrap, packing peanuts, or pieces of padded mailing envelopes
  • large binder clips or paper clips
  • meter sticks
  • clear plastic sandwich bags or quart freezer bags
  • paper towel sections

Our information-processing center—the brain—is the most shielded organ of the human body, protected from harm by many layers of tissue and bone. Its first line of defense is the cranium, or skull, which acts like a suit of armor, protecting the soft tissue of the brain from sharp blows that could easily damage it. The skull is made up of very strong bone that is difficult to crack or penetrate. In fact, many brain injuries resulting from bicycle or motorcycle accidents do not involve skull fracture. Instead, they involve disruptions of the soft parts of the brain and the layers of tissue that protect it.

Directly beneath the skull lie three membranes called meninges. The first membrane layer, just below the cranium, is the dura. It is tough and thick and restricts the movement of the brain within the skull. The middle membrane, the arachnoid, looks much like a spider web, which gives this layer its name. Closest to the brain is a membrane called the pia, which contains all the blood vessels that bring oxygen to the brain and carry waste products away from it. Between the arachnoid and the pia is a clear liquid called cerebrospinal fluid. This forms a liquid cushion, a kind of living shock absorber, between the soft brain tissue and the hard cranial bones.

Most brain injuries occur during a sudden impact, such as a crash involving a moving vehicle. In the instant before the impact, your brain, along with the rest of your body, is moving at a constant speed. Coming to a sudden stop causes a rapid change in your body's motion (i.e., a rapid deceleration). In a sudden deceleration, or slowing down, the various parts of your brain undergo the same process that the rest of your body experiences. The layers of protection beneath your skull help supply the force required to stop the motion of your brain in a sudden impact, preventing a collision of your brain with the hard interior surface of your skull.

The definition of force, first derived by Isaac Newton in 1686, explains why it is harmful for the brain to accelerate or decelerate too quickly. Newton said that the force exerted by an object is directly proportional to both the mass of the object and its acceleration (Newton's 2nd Law or F = ma). But the force applied during an impact also is dependent on time, because acceleration is the change in an object's speed, or velocity, divided by time (F = mv/t). The relationship between force and time in an impact is called an inverse proportion—when one quantity goes up, the other quantity goes down. One way to decrease the force of an impact is to increase the amount of time it takes for the impact to take place. This is the principle behind the engineering of a bicycle helmet.

Think of what happens when you jump from a height of several feet and land on the floor. The instant before you hit the floor, you are traveling at a moderate speed. When you strike the floor, your body decelerates and stops quickly. If you were to keep your legs straight and rigid, you would decelerate so quickly that you would risk breaking the bones in your legs. Instead, your natural reaction is to bend your knees, flexing them toward the ground, and lengthen the time it takes to decelerate—to slow your body's motion until it stops. You then land safely, with no injuries.

A similar scenario explains what happens to the brain during a crash involving a moving vehicle, even one moving relatively slowly. Most brain injuries sustained during bicycling accidents are caused by the rapid deceleration the brain experiences in the transition from motion to no motion. When the body suddenly comes to a stop, the brain is still moving, and it can strike the skull, causing damage to the cells and tissues that make up the brain's structures. The resulting change in mental acuity is called a concussion.

Bicycle helmets protect the head from physical injuries such as skull fractures. They also provide some degree of protection against brain injury. One important feature of a helmet is to slow down the rate at which the brain stops moving by increasing the time it takes for the brain to strike the hard inner surface of the cranium. This is accomplished by the sturdy foam helmet liners. In an accident, the head pushes against the foam and the foam compresses. This lengthens the time the head and brain take to come to rest. The brain may still make contact with the inside of the skull, but the force of the impact is reduced.

While helmets do absorb some of the force of an impact, they can only go so far in preventing brain injuries—concussions sometimes do occur in sports in which helmets are worn. Also, helmets cannot protect against whiplash, a fast jerking of the head, which can cause concussions as well. Many experts say that in addition to wearing a helmet during sporting activities, the best way for kids to avoid brain injury is to use good judgment and avoid extreme situations, such as performing flips while riding a bike.

In this activity, your students will build three models that represent heads (eggs), two of which will represent heads in bicycle helmets (foam). They'll test their models to see what degree of safety each provides by dropping them onto a hard surface from different heights.

Procedure Before the Lesson The Lesson
  1. As a class, watch the NOVA scienceNOW segment Brain Trauma. Use the segment to launch a short discussion about sports-related injuries. Find out how many students wear helmets or other protective gear when biking or playing sports. If you think it is appropriate, you also may ask if any students have ever experienced an injury such as a concussion. If students are comfortable talking about this subject, invite them to share experiences with the class.
  2. Introduce the activity by explaining that students will build models of bike helmets and will test the degree of protection their "helmets" provide to model "brains." In this activity, a complete egg, including the shell, represents the brain; layers of paper towel and plastic bag surrounding the egg represent the skull and meninges. An egg wrapped in a plastic bag plus paper towel represents a head.
  3. Hand out the eggs, paper towels, plastic bags, cushioning materials and Protecting your Brain: The Egghead Bicycle Helmet handout to student groups.
  4. Have students build and test model "eggheads," following the directions on their handout.
  5. When students have finished testing, make a class results table on the board or computer. Have each group of students enter their results (maximum height achieved with no injury/breakage for each helmet model). Have students compile average heights for each helmet model.
  6. Discuss other types of protective gear students might wear. You may be able to borrow gear such as shin guards, football helmets, and catcher's masks from the athletic department. How do these pieces of equipment protect body parts? How do they help alleviate the harmful effects of sudden impacts? For example, a catcher's mask has components that alleviate the effects of sudden deceleration—it has thick padding that covers the edge of the mask against the wearer's face, plus a series of stout metal wires that distribute the force of an incoming baseball or softball to every part of the mask instead of concentrating it at a single point. Knee and elbow pads are typically made of high-density foam that protects the bones by increasing the time that the force is absorbed (increasing the time of deceleration). You also may show students the interactive "Two New Helmets Do Battle Against Concussions" and discuss the kind of protection offered by the different parts of a football helmet.
  • Show the video clip Pressure Wave Interactions. Explain that the video shows a computer simulation of the "wave" of pressure that moves across the brain as it strikes the front of the skull during a car crash. Students should notice that the pressure or force of the impact builds steadily (turns from green to yellow to red) as the brain moves and decelerates from the top of the image toward the bottom. The video also shows a rebound of the pressure wave when the wave reaches the back of the head. The story discusses what nerve cells experience when exposed to this type of stress, such as disruption of cell membranes and damage to internal cell structures. Note: the video comes from a Sandia National Labs press release, "Sandia, UNM researchers show brain injury may occur within one millisecond after head hits car windshield"
  • Discuss some of the challenges physicians and coaches are faced with when someone has a possible brain injury. Why is it important to make a diagnosis quickly? Why might it be dangerous for an athlete to continue to play with a brain injury? As a class, discuss some of the technologies reviewed in the Flash Interactive Diagnosing Damage.
  • Have students investigate how bicycle helmets are tested, and explore some of the bicycle helmet safety standards established by organizations such as the Consumer Products Safety Commission. The Bicycle Helmet Safety Institute provides a good summary of this information.

Student Handout Questions

  1. Explain what the eggs, bags, and paper towels represent in your model. What does a cracked egg represent? The egg, including the shell, represents the brain. The bag and paper towels represent the skull and meninges, the structures that protect the brain. A crack in an egg in this activity represents a brain injury.
  2. In what ways were the "helmets" you designed and built good models of bike helmets? In what ways were they unrealistic models? The foam in the models serves the same purpose as the foam in a bicycle helmet. The models are unrealistic because the foam in a real helmet is denser and harder to compress. Also, bicycle helmets have several layers of protection (a hard outer shell) rather than one.
  3. What effect did additional padding have on the outcome of the egg drop? Why do you think padding made this difference? Eggs survived higher falls with padding; the padding increases the time of deceleration during the crash with the floor and also distributes forces.
  4. What other types of data could you collect to understand more completely the factors that affect the brain during a sudden impact? We could calculate the actual velocity of the object as it falls and the force upon impact. Calculating the force at impact would give a better understanding of the amount of protection a helmet provides. Then, we could determine how much force a given quantity of foam provides protection from.
  5. Bike helmets do reduce the risk of skull fractures and brain injuries. Yet, concussions are still common in sports in which helmets are worn. Why might this be the case? Helmets offer some protection, but they do not remove all risk of brain injury. Wearing a helmet does not necessarily ensure that you will not experience a brain injury, especially in an extreme situation, such as doing stunts while riding a bike, or performing other activities that generate significant injury-causing forces.

Use the following rubric to assess each team's work.

Excellent Satisfactory Needs improvement
Completing the Investigation
  • Students complete model-testing trials without difficulty.
  • Students show ability to apply information from their table to correctly calculate the percentages of prevention for each model.
  • Students understand how bicycle helmets are used to protect the brain from serious injury (cushion impact, distribute forces, increase time for deceleration).
  • Students need assistance in completing model testing.
  • Students show a limited ability to apply information from their table to correctly calculate the percentages of prevention for each model.
  • Students have some understanding of how bicycle helmets are used to protect the brain from serious injury (cushion impact, distribute forces, increase time for deceleration).
  • Students do not finish all required model-testing trials.
  • Students show no ability to apply information from their table to correctly calculate the percentages of prevention for each model.
  • Students have little understanding of how bicycle helmets are used to protect the brain from serious injury (cushion impact, distribute forces, increase time for deceleration).


The Protecting Your Brain activity aligns with the following National Science Education Standards (see

Grades 9-12
Physical Science

  • Motion and Forces
Life Science
  • Behavior of Organisms

Classroom Activity Author

Jeff Lockwood and WGBH educational outreach staff

Jeff Lockwood taught high school physics, chemistry, biology, astronomy, Earth science, and astronomical research during his 28-year teaching career in Tucson, Arizona. He also taught college astronomy for 17 years and has done educational consulting and curriculum development for Project STAR and AAVSO in Cambridge, NOAO, ASP, and others since 1987. He is co-author of high school level inquiry-based text programs—Investigating Astronomy (pending in 2009), Physics That Works (Kendall-Hunt), and Astrobiology: An Integrated Science Approach (It's About Time). Currently Jeff is employed by TERC in Cambridge, MA.

Teacher's Guide
NOVA scienceNOW: Brain Trauma

WebsiteBrain Trauma Quick Time or Windows Media Video
WebsiteDiagnosing Damage Flash Interactive
WebsitePressure Wave Interactions Quick Time or Windows Media Video
WebsiteTwo New Helmets Do Battle Against Concussions JavaScript Interactive