### The Physics of Irish Dance

Continuing on from last week, having covered ballet, we move on to Irish dance. While both are dance forms, completely different physics concepts apply to how dancers perform their seperate steps.

While ballerinas will use a plié to almost ‘explode’ off the ground as all their leg muscles contract in every single jump, Irish dancers build their entire style off never bending their legs or letting their heels touch the ground. Any ballet dancer can barely comprehend how a person could jump without bending their legs first … and you can try that yourself. Stand on tip-toes and see how high you can jump without bending your knees at all. It simply doesn’t work, yet jump extremely high with barely a bend!

So what’s going on?

It’s actually pretty simple physics behind it- centripetal force.

Centripetal force is the force for going round in circles. It’s generally illustrated by a ball-on-a-string example. If you’re swinging a ball around on a string it goes in a circle, but if you let it go, it will travel in a straight line from where you let it go. This is because the ball would naturally go in a straight line, but is acted on by the centripetal force of the string so that it instead accelerates by travelling around in a circular path (note that the term ‘accelerate’ in this instance does not necessarily indicate a change in speed, but rather a change in direction. Read more here if this is confusing). However, when you let go of the ball and string, there is no longer a centripetal force, so it heads off in a straight line, shown as the ‘path of inertia’ in the diagram.

Irish dancers exploit this principle to the maximum in the whole style and technique. When an Irish dancer performs one of their incredible jumps, they always swing their leg up to 90 degrees before leaving the ground. Once their leg is at 90 degrees and their other foot leaves the ground, they shoot in a straight line up high in the air, just like a ball being let off a string.

This means that their jump height is determined by the amount of force they put into swinging that leg up in the first place, rather than by the potential energy of a plié that ballet demands.