preparation for this lesson, place a car in a safe lcation for the
students to measure the tire footprints and pressure. In case of bad
weather, find a covered location. Be sure to measure the tire footprint
and the pressure (in PSI) of each tire ahead of time, so that you will
be able check the accuracy of students' measurements. Also, check the
accuracy of your calculation by comparing to it to the weight of the
car listed on the sticker inside the driver's door or in the vehicle
By the end of the day, data may change because air has leaked out of the tires while students were using the tire gauge. For safety, check the tires before driving home.
The table below represents sample data from a 2002 Toyota Camry.
| ||Length of Tire|
|Area of Tire Footprint|
in Each Tire (psi)
|Calculated Weight Tire|
is Supporting (lbs)
|Driver-Side Front Tire||5 3/4||6 1/2||37 3/8||33||1233 3/8|
|Driver-Side Rear Tire||5 1/8||6 1/4||32 1/32||32||1025|
|Passenger-Side Rear Tire||5 1/2||5 3/4||31 5/8||34||1075 1/4|
|Passenger-Side Front Tire||6||6 1/2||39||32||1248|
To introduce the lesson, discuss weight and pressure with the class.
Ask, "Would it hurt more if a cat or a cow stepped on your toe? How
much weight would be on your toe?" Have students speculate on why the
cow hurts more than the cat. Lead the students into a discussion about
pounds per square inch and distribution of weight on each of the
animal's four feet. Ask, "What else is measured in pounds per square
inch (psi)?" [Tire pressure.] Also, ask questions that might remain
unanswered until the lesson is complete:
- About how much air pressure is in a car tire?
- How do you measure it?
- Would knowing the pressure in a car tire tell us anything about the weight of the car?
- Why do large vehicles have larger or more tires?
These questions are meant to generate a discussion that will perk
the interests of the students before doing the activity. They will
discover answers as the lesson proceeds.
Students may have learned in science that Pressure = Force ÷ Area.
This means that pressure is a measure of how the applied force is
distributed over an area. In our case, the force is the earth's
gravitational force on the car (i.e., the weight of the car.) This
weight is distributed over the ground by the surface area of the tire
touching the ground.
Arrange the groups of four students so that there are multiple
levels of students in each group. Give each student in the group a job:
- Recorder: records all information on the
Activity Sheet and makes sure that each student completes their own
Activity Sheet. Recorder is a good job for someone who has low math
- Area Measurer: measures the footprint of the tire and reports information to the recorder
- Pressure Measurer: uses a tire gauge to measure the air pressure in each tire and reports the information to the recorder
- Multiplier: does the multiplication in the chart without a calculator.
All students in the group should assist each other when needed and verify accurate results on all measurements and calculations.
Have students answer the Questions 1 and 2 on the How Much Does a Car Weigh? Activity Sheet.
Give students directions for how to find the data for the table in
Question 3. Explain that they will use two pieces of poster board to
find the length and width of the tire's footprint. They should slide
one piece of poster board in front and one behind the tire, or one on
either side, to see where the tire is touching the ground. They need to
be sure that the poster board is straight at both ends. Students will
use a ruler to measure the distance between the two pieces of poster
board. (See photo below.) Because using a tire gauge can be
challenging, they should have have a second group member check the
pressure before recording it. For calculating the area, they can assume
the footprint is a perfect rectangle, and to find the weight that the
tire is supporting, they can multiply the area of the footprint by the
When all the measurements and calculations are complete, have
students complete Question 4. Then, discuss as a class the actual
weight of the car before having students complete Question 5.
Prompt students to review what they have learned with a question
they must answer on paper before they can leave the class. The
following are possible questions that could be asked.
What have you learned today about:
- Weighing cars?
- Using a tire gauge?
- Tire footprints?
- Distribution of weight?
- Multiplying with fractions?
- Anything else?
Other questions might deal with how the students worked together in groups.
What have you learned today about:
- Working as a team?
- Being responsible for your job?
- Helping others?