This activity appears in this month's edition of The Helix magazine. This month's magazine will take you on a journey through the weird and mysterious world of a simple drop of pond water. In a mere millilitre of this innocuous substance there lives a teeming world of microscopic alien life forms - if only you know what you're looking for. We also take a look at the new Australian Synchrotron, learn about biomimetics and discover the secret to making good popcorn. To subscribe to The Helix magazine visit CSIRO's Double Helix Science Club website.
You will need
One car
One adult car driver
Four pieces of paper
Sticky tape
A ruler
A calculator
Tyre pressure gauge
What to do
Using the tyre pressure gauge, measure the air pressure of one tyre. Tyre pressure is measured in different units - the most common is psi (pounds per square inch) or kilopascals. This activity is easiest if you have a tyre gauge that measures in metric units, kilograms per square centimetre (kg/cm 2 ). If you don't have a tyre pressure gauge at home, try your local service station.
To measure the car's footprint, which is the area of the tyre in contact with the ground, first ask the driver of the car to make sure the handbrake is on.
Place four pieces of paper around the tyre; at the back, front and each side. The pieces of paper should overlap and be wedged in as tightly as you can.
Tape the paper down on the ground and step away as the driver moves the car off the paper. You will be left with a rectangle area inside the paper which is the car's footprint.
Calculate the area of the car's footprint in cm 2 by multiplying length by breath.
Then work out the weight on one tyre; which is the footprint area multiplied by that tyre's pressure. Then multiply this number by four. Your answer is the approximate weight of your car. If you used a metric gauge, the answer should be in kilograms. If you used a pump that measures psi, you need to measure the footprint area in inches, and then convert your final answer to kilograms using this conversion: one pound = 0.45 kilograms. As a guide the average 4WD family car weighs about 2 tonne or 2000 kilograms (kg). So if you car is smaller than that, the answer you get should be less than 2000 kg.
What's happening?
When you pump up a car tyre, you increase the pressure of the air inside the tyre by increasing the number of molecules inside the tyre. It is air in the tyre that carries the total load of the vehicle and passengers. A standard 4WD vehicle weighs about 2 tonnes and that's without any passengers or gear. So there's a lot of weight on the four tyres. It is essential to regularly check your tyres to make sure your car drives safely.
Even though we can't feel it, air is constantly pressing down on us with a tremendous force.
Atmospheric air pressure is created by the weight of the earth's atmosphere. Although we can't see air, the gas molecules still have mass, and gravity acts upon it.
The air pressure changes daily due to the heating and cooling of the earth's surface. When air gets warm, it expands, becoming less dense, and therefore pushes with less pressure.
Applications
We can measure changes in atmospheric pressure by using a barometer. A "rising" barometer indicates increasing air pressure; a "falling" barometer indicates decreasing air pressure. As a very loose rule, a high-pressure area will be clear, and a low-pressure area will be cloudy and rainy.
We use air pressure all the time when we breathe. When our diaphragm moves down, air is pushed into our lungs from the outside, expanding the volume of the chest cavity. The diaphragm doesn't "pull" air in; it expands the volume of our lungs, and the air pressure fills the volume.
How can you weigh something as big as a car without using an extremely large set of bathroom scales?
Place four pieces of paper around the tyre; at the back, front and each side. The pieces of paper should overlap and be wedged in as tightly as you can.
