PHYS 1000

Activity: Measuring the Weight of Your Car

Objective:To measure indirectly the weight of a car by measuring the pressure in the tires and the surface area of the tires in contact with the road.

Background: Pressure is defined as force per unit area. If we know the force that is being exerted on an object and know the area over which that force acts, we can calculate the pressure by using the definition. For example, if a force of 100 lbs is exerted over an area of 10 square inches, then the pressure resulting from that force is

Pressure = Force/Area = 100 lbs/10 in² = 10 lbs/in².

We can use the same relationship to determine the force if we know the pressure and the area. Simply rearranging the equation, we get

Force = Pressure x Area, or F = P A.

The rims of the wheels of a car are held off the ground primarily by the force of the air in the inflated tires. The tires theselves are elastic and often have steel belting which tend to support the car also, but most of the support results from the air pressure.

In this activity you will approximate the weight of your car by assuming the entire weight is supported by air pressure in the tires. The force of each tire on the ground is related to the pressure in the tires and the area of the tires in contact with the ground. The force of gravity pulls the car down and the road pushes up with the same strength. The force that the road pushes up on each tire is simply the pressure of that tire times the area of contact the tire makes with the ground.

The sum of the forces on the four tires of the car m.ust be equal to the weight of the car (otherwise there would be a net force on the car).

Worksheet Be sure to include images showing your tire footprints. Total Force: W= lbs wwwv 1) List the actual curb weight of the car: Actual Weight = W: [Jag 2) Calculate the percent djembmen your value of the car's weight and the listed weight by using the formula I W W l x measured W 3) Comment on your values and any discrepancies. What do you think are the sources of error? (Note: if you are deviating significantly from this value, you should re-check your calculations.) Percent Difference = 100 Worksheet Be sure to include images showing your tire footprints. Total Force: W= lbs W 1) List the actual curb weight of the car: Actual Weight = W: [Rs 2) Calculate the percent mm your value of the car's weight and the listed weight by using the formula I W W | x measured W 3) Comment on your values and any discrepancies. What do you think are the sources of error? (Note: if you are deviating significantly from this value, you should re-check your calculations.) Percent Difference = 100