Ruler Break

Hand of fury – see the amazing ruler-breaking strength of air.

Australian Curriculum links

  • Science > Physical Sciences > Year 4 > ACSSU076
  • Mathematics > Measurement and Geometry > Year 6 > ACMMG137

You'll need

  • old plastic ruler (not your favourite ruler because it will get broken)
  • table edge
  • large sheet of newspaper

Try this

  1. Place the ruler on the edge of a table. Leave about 8-10 cm of the ruler hanging over the edge of the table.
  2. Make sure no one is standing close to you and then give the ruler a good karate chop.
  3. Set your ruler up again, this time covering the end that is on the table with a large piece of newspaper.
  4. Give the end of the ruler another good karate chop while letting loose your most intimidating “Ki-Yah”!

Further investigation

Try this activity using different sized sheets of newspaper. How big does the sheet of newspaper have to be for a ruler to snap? If you know the area of the newspaper, you can calculate the total force of the air pressure pushing down on the newspaper.

First, you can calculate the area of each sheet of newspaper by multiplying the length of the sheet by the width of the sheet.

Area = length (cm) × width (cm)

Now calculate the mass pushing down on the paper in kilograms by multiplying the area of the sheet by the air pressure. We already know that: air pressure = 1kg/cm2

Mass pushing down on the paper (kg) = Area (cm2) × air pressure (kg/cm2)
= Area (cm2) × 1kg/cm2

Force is measured in Newtons (N). To calculate the force on the paper, multiply the mass pushing down on the paper in kilograms by the acceleration due to gravity.

Force (N) = mass pushing down on the paper (kg)
× acceleration due to gravity (m/s2)
= mass pushing down on the paper (kg)
× 9.8m/s2

What's happening?

You may not have noticed this before, but air has weight, just like a bag of sugar, just like an elephant and just like you. Air is pushing down on us all the time and it was pushing down on the ruler the first time you karate chopped it and sent it catapulting across the room.

A ruler has a small surface area (surface area just means how big something’s surface is) so the amount of air that is pushing down on the ruler isn’t enough to hold it on the table. A piece of newspaper has a very large surface area compared to a ruler so a much larger amount of air is pushing down on it. The weight of the air pushing down on our piece of newspaper is more than enough to hold our ruler in place, letting us snap it into a whole bunch of different pieces.

Air pressure is measured in Pascals (Pa). Atmospheric pressure is about 101300 Pascals, or 101.3 kPa (kilo-Pascals), which is about 1 kilogram per square centimetre. Imagine something that weighs about 1 kg (eg. a bag of sugar or flour or a litre of milk). Now imagine balancing that 1 kg object on your thumb nail. Your thumb nail has an area of about 1 cubic centimetre, so the force you are feeling is about the same as the force of the air in the atmosphere pushing on every square centimetre of the newspaper in this activity.

Real world links

A suction cup works because the air inside the cup is squeezed out and the atmospheric air pressure holds the cup in place. There needs to be an air-tight seal between the suction cup and the thing it is stuck to, for example a glass window. When the air inside the suction cup is squeezed out, the area inside the cup becomes a vacuum, an area of very low air pressure. The air surrounding the suction cup is still at normal atmospheric pressure of about 101.3kPa and it pushes in on the suction cup, keeping it squashed against the glass and holding it in place.