Engineering Car Crash Safety with Newton's Third Law

Background Information for Teachers

Newton's third law of motion states "For every action, there is an equal and opposite reaction" ("reaction" here refers to a force, which is a push or pull). For example, when you put a book on a table, the weight of the book pushes down on the table (the action). The table pushes back on the book (the reaction), with an equal force that acts in the opposite direction. This force is what prevents the book from falling to the floor (Figure 1).

Image Credit: Ben Finio, Science Buddies / Science Buddies
Figure 1. Action and reaction forces between a book and a table.

This concept can be confusing for students, and several misconceptions exist. For example, students might ask why the two equal and opposite forces don't just cancel each other out. This is because the two forces act on different objects (e.g. in Figure 1, the action force acts on the table, and the reaction force acts on the book). Equal and opposite forces only cancel each other if they act on the same object. Students may associate exerting a force with things that can move, such as a person pushing a box across the floor, or a snow plow pushing a pile of snow. It can be difficult to grasp how an inanimate object can push. For example, if you say "When you push on a wall, the wall pushes back on you," students might ask "How can a wall push if it can't move?" The lesson will help you address some of these misconceptions with students.

In this lesson plan, you will illustrate Newton's third law by crashing model cars that the students build themselves. When two cars crash at a high enough speed, both cars are usually damaged (Figure 2). For example, think about one car rear-ending another car that is stopped at a red light. When the cars collide, the moving car exerts a force on the stationary car, which can damage the rear end of the stationary car. However, the moving car comes to a stop, and its front end is also damaged. You can observe that something must have exerted a force on the moving car.

Image Credit: Science Buddies
Figure 2. Example of a car crash where one car rear-ended another and both cars are damaged.

According to Newton's third law, when the moving car exerted a force on the stationary car, the stationary car exerted an equal and opposite reaction force back on the moving car. If a single car crashes into a sturdy, stationary object like a wall or building, the stationary object also pushes back on the crashing car with an equal and opposite force (Figure 3). Again, you can observe the existence of this force because you observe the car slow down and become damaged.

Image Credit: Ben Finio, Science Buddies / Science Buddies
Figure 3. Diagram of action and reaction forces for a car crashing into a wall.

Cars have bumpers designed to protect the body of the car from minor damage during low-speed collisions. In higher-speed collisions, the impact forces will be higher (this comes from Newton's second law—see Additional Background section). The "crumple zone" protects the occupants of the car, because the body of the car itself absorbs the force of impact and crumples (in Figure 2, notice how the trunk of the yellow car is smashed, but there is little damage near the door and rear passengers seats). In this lesson, your students will design and build toy cars, and perform crash tests on their cars by rolling them down a ramp into a wall. They will use the engineering design process to design and build bumpers to protect the main parts of their car from damage, and use their knowledge of Newton's third law to explain what they observe.

Note: this lesson is written for the NGSS Performance Expectation related to Newton's third law of motion. You can also adapt this lesson to teach about Newton's first and second laws of motion, or kinetic and potential energy. See the Variations section for details.

Additional Background Links

• Newton's Third Law, The Physics Classroom
• Bumper Cars, Annenberg Learner
• How Crumple Zones Work, HowStuffWorks
• Push Harder—Newton's Second Law, Science Buddies
• Engineering Design Process, Science Buddies

Prep Work (5 minutes)

• Set up one or more ramps that your students can use to crash-test their cars by rolling them into a wall. You can do this by propping one end of a wooden board or large piece of cardboard up on a box or stack of books, and aiming the other end at the wall (Figure 4). In order to give your cars enough speed, your ramp should be at least three feet long and one foot high. Starting the cars at the same position on the ramp (as opposed to letting students push the cars into the wall) will allow students to perform their crash tests at approximately the same speed each time.
• Note: this exercise works best if there is visible damage to your students' cars, such as the chassis being crumpled or the wheels/axles falling off. If there is no damage to the cars, they are either too light or not going fast enough. In general, a bigger ramp will work better. If you don't have room to make a big ramp, you can increase the weight of the cars by taping weights to them, like coins or large rubber erasers.

Image Credit: Ben Finio, Science Buddies / Science Buddies
Figure 4. Recommended minimum dimensions for ramp.

Engage (45 minutes)

1. Show your students Figure 2. Prompt students to think and ask questions about what happened in the picture to cause the result that they see. For example, what do you think was happening immediately before this picture was taken? How could we explain what we see in this picture with physics?

Image Credit: Science Buddies
Figure 5. Two cars after an accident.

2. Explain that today students will build and crash-test their own cars. Now they will get started building their own cars in small groups. The goal for now is to quickly build a working car that can smoothly roll down a ramp, not to optimize or improve the car.
   1. Introduce the materials your students have available to work with.
   2. Show how students can build an axle, which is generally the most difficult part of the car to build. You can show your students this video, or demonstrate how to build one yourself.
      

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   3. Each individual student should make a list of materials and sketch of their car design on their worksheet.
   4. Students within each group should compare their designs and agree on what design they will build. It can be a single design, or a combination of two or three students' designs. Students should be careful to pick a design they think will work the best, not just what they think looks the coolest!
   5. Each group should build a prototype of their car.
3. Let each group roll their car down the ramp and crash it into the wall. The whole class can observe each test. Each group should record observations about what happened to their car on their worksheets. What happened during the crash? What does the car look like after the crash?
4. Have a brief discussion about Newton's third law of motion; but don't start out by telling students what the law says!

   What made our cars come to a stop? What caused the damage to the cars?

   The cars came to a stop because they crashed into a wall. The collision with the wall caused damage to the car.

   How does the wall stop the car? Can a wall "push"?

   Students might say that a wall cannot push, because they associate the act of "pushing" with things that can move (like a person or a snow plow). Let students experience how a wall can push.

   1. Ask students to press their hands against a wall. What do they feel?
   2. Does what they feel change if they press against the wall harder? What if they lean against the wall with both hands?

   Did you answer change? Can a wall push?

   Yes; even though a wall cannot move, it can still push. You can feel the wall pushing back on your hand when you press against the wall. When you press harder, the wall presses back harder.

   Now apply what you just learned to your car instead of your hand. What happens when the car pushes on the wall?

   The wall pushes back on the car.

   What force makes the car stop and causes damage to the car?

   The force of the wall pushing on the car.

5. Explain that students just demonstrated Newton's third law of motion. Formally, the law states that "For every action, there is an equal and opposite reaction." "Reaction" refers to a force (a push or a pull). This means that when you push on the wall, it presses back on you with a force that is exactly equal in size, but points in the opposite direction. Now students will design a bumper to protect their car during a crash. It is OK if the bumper itself is damaged during the crash and needs to be repaired or replaced. The goal is to protect the main parts of the car (chassis, wheels, and axles) from damage.

Explore (45 minutes)

1. As a group, students should repair any damage to their car from their crash test, or make a new car if needed.
2. Prompt students to brainstorm how they could build a bumper to protect their car from damage when it crashes into the wall. The design should focus on a bumper that is an addition to the existing car, and does not require totally rebuilding the car. Each student should sketch a design and make a list of materials on their worksheet. They will need to apply their knowledge of Newton's third law to the design of their bumper (when the car pushes on the wall, how will the wall push back on the car?). They will also need to think about the properties of different materials that they have available. Would some be better than others for building a bumper?
3. Within each group, students should compare their designs and decide what they will build. Again, they can pick the best parts of different designs and combine them to make a single, better design. A solution needs to be tested, and then modified on the basis of the test results, in order to improve it.
4. Each group should build a prototype of their bumper and attach it to their car. When they are ready, they can come to the ramp to test it, observe what happens, and record their observations.
5. Bring the class together for a discussion. Explain that the engineering design process is iterative. It is not like a math problem where there is usually a single "right answer." Multiple designs might all be good, and there might be an even better design that no one has thought of yet. Sometimes, ideas can look great on paper, but things do not always go as planned or do not work on the first try. Engineers usually have to repeatedly test and re-design something to make it better.
6. As a class, decide how you will test the bumpers to compare them and find out which ones work the best to protect the car it is attached to. There is more than one way to do this, which may depend on the available height of your ramp. For example:
   1. Which car can survive a crash from the highest starting position on the ramp, with no damage to the chassis or wheels/axles (works well for larger ramps)?
   2. Which car can survive the most repeated crashes from the same height, with no damage to the chassis or wheels/axles (works well if your ramp height is limited)?
7. Now, students will need to test and improve their bumpers in order to meet this goal. While students can make minor modifications and repairs to their cars if necessary to affect how they attach the bumper, the main emphasis at this point should be on designing the bumper and not totally redesigning the car. Any modifications should be documented. Each group should go through at least two iterations of their bumper and record their observations/improvements on their worksheets.
8. After all groups have finalized their designs, perform the final tests one group at a time, while the whole class watches. Record the results for each group (e.g. the number of crashes or highest height on the ramp it took to damage their car). How do the results compare to when they crashed cars without bumpers?

Reflect (15 minutes)

1. Discuss the results of your engineering project as a class. Which bumper designs worked the best? Did they have anything in common? Did the design of the car itself affect the performance of the bumper?
2. Circle back to the picture you saw at the very beginning of class.

   Can you explain what you see in the picture in terms of Newton's third law?

   The picture shows two cars that are damaged because the front of one car crashed into the back of the other one. Both cars are damaged because they exerted equal and opposite forces on each other when they crashed.

   Can you explain what happened to your car when it crashed in terms of Newton's third law?

   When your cars crash into the wall, they exert a force on the wall. The wall exerts an equal and opposite force back on the car, which can break the car. A bumper helps absorb that force and prevent damage to the main parts of the car.

Assess

• Collect students' worksheets and use them to assess their use of the engineering design process. For example:
  • Did students make clear sketches for their design ideas?
  • Did students record their observations?
  • Did students make a causal link between their observations and changes they could make? For example, "The bumper fell off when it crashed, so I will use more tape to attach it."
• Ask each group to make a poster or do a brief presentation about their car and their bumper design. Ask them to include Newton's third law in their poster or presentation.

Make Career Connections

Discussing or reading about these careers can help students make important connections between the in-class lesson and STEM job opportunities in the real world.

Automotive Engineer

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Career Profile

Automotive engineers design all the systems, like crumple zones, airbags, bumpers, and seat belts, that help keep passengers safe during a real car crash. If you liked building and testing cars, and thinking about how to make them safer during a crash, you should learn more about automotive engineering Read more

Mechanical Engineer

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Career Profile

Did you like building and (crash) testing things, but maybe you don't like cars that much? Mechanical engineers build and test many other types of machines, from airplanes to boats to spaceships. They use the engineering design process to improve these machines and make them safer for people. Read more

Lesson Plan Variations

• This lesson is written for an NGSS Performance Expectation regarding Newton's third law of motion. You can also adapt it to be about other physics concepts:
  • Newton's first law of motion: an object in motion remains in motion, and an object at rest remains at rest, unless acted upon by an outside force. In other words, the car will keep rolling on the floor until it is acted upon by an outside force by a collision with the wall (or until friction slows it down more gradually).
  • Newton's second law of motion: the net force on an object is proportional to the mass of an object times its acceleration. How do the car's mass and its velocity just before impact affect the force on the car and the resulting damage? (Note that acceleration is a change in velocity per unit time).
  • Kinetic energy, conservation of energy, and potential energy. When the car is raised up the ramp, it has gravitational potential energy. When you release the car and it rolls down the ramp, the potential energy is converted to kinetic energy. When the car crashes into the wall, the kinetic energy is converted to other forms, like heat, sound, and elastic potential energy (the energy stored in stretched/bent/compressed materials).
• If you are doing this lesson as part of a unit about forces, and have introduced force diagrams or free body diagrams to your students (drawings where forces are represented by arrows), ask your students to draw a diagram with arrows representing the forces acting on the car and the wall.
• Instead of crashing cars into a ramp, let students crash their cars into each other. Experiment with different collisions, e.g. rear-end collisions (one car is stationary on the ground in front of the ramp), side impact or "T-bone" collisions (one car is placed sideways in front of the ramp), or head-on collisions (set up a second ramp facing the first ramp and roll both cars down at the same time). How does damage to the cars vary in different cases? Can students build a bumper to protect the car from crashes in all directions?
• There are other ways to power a toy car instead of rolling it down a ramp, including balloons, rubber bands, and the wind! Check out these Science Buddies resources on other car-related projects, including additional NGSS lesson plans! Note: some of these cars may not go fast enough to work well with the crash-test lesson plan, but the cars used in this lesson can usually easily be altered to use a different power source .
  • Balloon Car (NGSS Lesson Plan)
  • Solar-Powered Car (NGSS Lesson Plan)
  • Newton's Second Law (NGSS Lesson Plan)
  • Rubber Band Car (fun science activity)
  • Wind-Powered Car (fun science activity)
• Let students design cars to carry action figures or small dolls, and design seat belts to keep the figures safe during a crash.
• Add weight to the cars using coins, large rubber erasers, small rocks, etc. Are the bumpers strong enough to withstand the impact with added weight? If not, can students keep using the engineering design process to improve their bumpers?