Replica Gun Discussion on Liveline

Created by Grok.

A discussion on RTÉ Liveline today was about "Airsoft" type guns on public display in shops. Under the Firearms Acts 1925, revised to 2023, these guns must have a muzzle energy of less than 1 joule to avoid being classified as firearms requiring a license.

What's a joule?

It's the Système International (SI) unit of energy or work done.

Energy = power x time

So 1 watt for 1 second = 1 x 1 = 1 joule.

Think of a 1 watt bulb turned on for 1 second; that consumes 1 joule of energy.

Since energy can't be created or destroyed (except in nuclear reactions where there are mass-energy conversions), "consumes" doesn't mean the bulb destroys the energy, it's simply converted into another form of energy, light and heat. For an incandescent bulb, 95% of the energy is heat.

Another example of a joule is the work required to lift a weight. Work in physics has a very specific meaning. It's defined as something which occurs when a force moves a body through a distance. When an object is lifted off the ground, a force is required to counteract the force of gravity. 

The work done W when a force moves a mass m a distance h above the ground is defined as:

W = mgh

if m is 1 kilo and h is 10 cm (0.1 m)

and g is the acceleration due to gravity; approximately 9.81 metres per second per second

W = 1 x 9.81 x 0.1 = 0.981 or approximately 1 joule.

So that's the amount of energy an Airsoft pellet is limited to; the potential energy of a bag of sugar, raised 10 cm off the ground. (or 100 g, raised 1 m off the ground).
Contrast that with the energy of a rifle bullet, which is approximately the potential energy of a brick raised to the height of a chimney on a two story house.