# Engineering Car Crash Safety with Newton's Third Law

## Summary

6th-8th
Group Size
2-3 students
Active Time
2 hours
Total Time
2 hours
Area of Science
Physics
Mechanical Engineering
Key Concepts
Newton's third law of motion, engineering design
Credits
Ben Finio, PhD, Science Buddies

## Overview

Combine Newton's third law of motion with engineering design in one fun lesson! Your students will learn about equal and opposite reaction forces as they design and build a bumper to protect a toy car during a crash.

## Learning Objectives

• Use the engineering design process to iteratively design, build, and test a product
• Apply knowledge of Newton's third law to explain what happens in a car crash

## NGSS Alignment

This lesson helps students prepare for these Next Generation Science Standards Performance Expectations:
• MS-PS2-1. Apply Newton's Third Law to design a solution to a problem involving the motion of two colliding objects.
• MS-ETS1-4. Develop a model to generate data for iterative testing and modification of a proposed object, tool, or process such that an optimal design can be achieved.
This lesson focuses on these aspects of NGSS Three Dimensional Learning:

 Science & Engineering Practices Disciplinary Core Ideas Crosscutting Concepts Science & Engineering Practices Developing and Using Models. Develop a model to generate data to test ideas about designed systems, including those representing inputs and outputs. Disciplinary Core Ideas PS2.A: Forces and Motion. For any pair of interacting objects, the force exerted by the first object on the second object is equal in strength to the force that the second object exerts on the first, but in the opposite direction (Newton's third law). ETS1.B: Developing Possible Solutions. A solution needs to be tested, and then modified on the basis of the test results, in order to improve it. ETS1.B: Developing Possible Solutions. Sometimes parts of different solutions can be combined to create a solution that is better than any of its predecessors. Crosscutting Concepts Structure and Function. Structures can be designed to serve particular functions by taking into account properties of different materials, and how materials can be shaped and used.

## Materials

This is an engineering design project, so there is not a specific list of required materials. You can make different materials available to your students, or allow them to bring materials from home. In general, recycled items, craft supplies, and office supplies work well. At a minimum, make sure your students have materials available to make and connect the main parts of the car.

• Wheels: round objects like bottle caps or CDs, or you can cut your own wheels from foam board
• Axles: straws, pencils, wooden skewers, paper rolled into tubes, etc.
• Chassis (body): plastic bottles, cardboard boxes, Styrofoam®, cardboard, etc.
• Other construction supplies: scissors, tape, glue, rubber bands, paper clips, etc.

You will also need materials to build a ramp to roll the cars down:

• Wooden board, large piece of corrugated cardboard, or foam core board, at least 1 foot wide and 3 feet long
• Small box or stack of books to prop up one end of the ramp, at least 1 foot tall

## Background Information for Teachers

This section contains a quick review for teachers of the science and concepts covered in this lesson.

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).

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.

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.

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.

## Lesson Plan Variations

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