The Law of Inertia



An edited version of an individual lesson plan developed by

Albert R: Palazzo, East Meadow High School, East Meadow, Long Island, NY

Courses for Which Lesson is Intended:

Physics classes.

Types of Teaching/Learning Activities Employed in this Lesson:

Students are asked to explain the law of inertia and apply it to the use of seat belts and airbags in automobiles.

Category that Best Describes this Lesson:

Behavior of scientists.

Social issues.

Ethics/Values Issues Raised by this Lesson:

Risk assessment, persuading others not to expose themselves to excessive risk.

Lesson Plan

This lesson begins with a discussion of the law of inertia ("a body at rest remains at rest and a body in uniform motion continues moving uniformly unless acted on by a net force"). Next, the law of inertia is applied to a specific context, the use of seat belts and airbags in automobiles. It is well known that automobile accidents often cause serious injuries or deaths as a result of the momentum of cars crashing into other objects. Devices such as seat belts and airbags can help by restraining or cushioning forward motion and lessening impact forces during collisions or other sudden vehicle decelerations. In addition, head restraints can prevent injuries such as "whiplash" due to rear end collisions.

After discussing these ideas, students are invited to comment on the following scenario:

A physics teacher is a passenger in a car driven by a colleague. As they are about to set out on a lengthy trip, the physics teacher notices that the driver is not wearing his seat belt. Should the teacher say something to the driver about this? If so, what should he say?

Suppose the physics teacher reminds the driver about the seat belt, but the driver replies, "I just don't feel safe wearing a seat belt. I've heard about some accidents in which people were killed because they couldn't get out of their belts. Besides, I don't really see the point. If the car goes forward, I go with it; if it stops, I stop. What can a seat belt do about that? Nothing. Isn't this a free country? We should be able to choose--and I've made my choice." What, if anything, should the physics teacher say now?

Students are asked to write out responses to these questions and then to discuss their answers with the rest of the class.


Students are likely to have a number of different views on these matters. Although the law of inertia is a settled matter, the question of whether or not to wear seat belts is not. Here we have to make choices. The question is, on what basis should such choices be made? Physics can help us understand possible consequences of the choices we make. It cannot by itself settle the questions of responsibility the use of automobiles raises--questions about the responsibilities designers and manufacturers have in producing reasonably safe automobiles for consumer use, the responsibilities governments have to establish and enforce safety standards to protect consumers, responsibilities consumers have to protect themselves, and responsibilities we have to urge those with whom we travel to "buckle up".

An understanding of basic principles of physics can help all of the above parties wrestle with their responsibilities. Does this give physics teacher's special responsibilities to help others understand the likely consequences of choosing not to use seat belts and air bags?

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