## Science of Curling

Curling is an ancient sport and has been a part of Winter Olympics for a while now. Why is it becoming a topic of discussion here? Well, like most other sports, there is a good amount of physics associated with it. Curling is essentially an application of friction and its manipulation. But there is a slight mismatch between theory and observation. A mismatch that plays a huge role in the game.

## How Does the Arrangement Work?

Two competing teams play a match. Each team, comprising 4 players, gets 8 throws in the game with each player throwing twice. The teams attempt alternate throws of the curling stone, on a sheet of ice. The motive of the sport is to target the stone at the center of the concentric circles, which are called house (very much like a dart board, except everything here is on the ground). So, closer the stone ends up to the house, better the scores for the team.

## Curling Stone

The curling stone is a polished marble stone made of granite. It is brought especially from a Scottish coast or Trefor Quarry in Wales to maintain consistency of the surfaces in contact. The bottom of the stone is such that the surface in contact with the floor is not a disc but an annular ring. The top is fitted with a handle for slider’s convenience.

## The Throw

Now, the throw has to really be a disciplined one. When the stone is slid, it is also given a rotation depending on the required trajectory to stop at the target. This is where a difference in theory and practice prevails.

## What’s Expected?

If the stone is rotated in a clockwise sense, it should curl leftwards. When it is rotated in an anticlockwise sense, it curls rightwards. Why? Because an object moving forward is essentially in a state of topple (but with entire bottom surface grounded) with a line of ‘Normal Force’ acting away from the center of mass towards the leading edge. It is a transition state of the object between properly grounded to entirely lifted. Although object appears grounded, the normal force is more on the leading edge of the stone compared to the lagging one. Therefore, the friction acting on the leading part is more. So, if the leading edge moves rightwards (i.e. clockwise sense of rotation) relative to the floor surface, the stone should curl leftwards. Under normal circumstances, this is what’s observed with a moving, rotating object.

But what is observed in curling is the exact opposite of this. At first sight, this is very deceiving to the eye. The only possible way to explain the observation, since no other horizontal force plays, is if the friction acting on the leading edge is actually less compared to the lagging edge. But how?

## The Floor

This is where the catch lies. The floor, maintained at -5 on Celsius scale, is sprinkled with water droplets. This is where the entire manipulation of friction comes into play. Let’s first explain the ambiguity. Had there been no droplets, the leading part of the curling stone would experience more friction. But since it moves over the droplet, the friction it experiences is less compared to the friction experienced by the lagging edge of the stone for which the surface with same coordinates is rougher than the edge leading it. So, the ambiguity is not an ambiguity after complete knowledge of the system.

There is another theory called ‘The Scratch Theory,’ which offers another possible explanation to this.

## Manipulation of Friction

If the throw is somehow not that great, still good scores can be secured. This is where the sweepers, who have to be proactive, come into play. They are members of the team who move with the stone and sweep the floor vigorously to arrest or drive the stone towards the House, which can be seen in the image.

Apart from manipulating their throws, a team can knock off other team’s stone farther from the house. It makes the sport even more interesting. In fact, whenever a head-on collision between two stones takes place, sweet memories from ‘Conservation of Momentum‘ come back to life. All school kids who are in love with physics should, without second thoughts, take a look at this sport.

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