Physics 3401 Scientific Investigation April 8, 1996

Why a Curling Stone Curls

.

David D. Babcock


Abstract: The key concepts to why a curling stone curls are the forces of friction bewteen the surface of the stone and the surface of the ice. Specifically 'edges' parallel to the motion of the stone are of interest. At any particular point while the rock is rotating one edge is opposing the motion of the rock while the other edge favors the motion. The dependence on curl and friction is examined futher in the following.

Introduction: Will examine why a curling rock curls, through the use of NIH image, the intuitive mind of the curler and by examining the forces acting on between the ice and the rocks we develop an explanation.

Procedure:

  1. Set the Video camcorder up at one end of the ice directly in line with the centre line. (The centre line being the line that bisects the curling ice lengthwise.) Making sure the camera view can see at least both sides of the eight foot circles (see diagram below).

  2. Construct a scale along the side of the ice, so that the position of the rock will be able to be determined using video analysis. Here we placed eight curling stones starting from the top of the rings on both sides of the ice leaving them 0.93 m from the side and 1.0 m apart. The rest of the rocks were placed 2.0m and then 4.0m apart when it became appropriate. See Diagram #1

  3. Started recording and got the curler to throw a stone with a clockwise rotation. If successful, dictated by whether or not the stone came to rest within the field of vision of the camera, we kept the recording.

  4. Repeated 3. for a stone rotating in the counter clock wise direction.

Diagram #1
Results:

Utilising the NIH image software, making sure to set the scale each frame, we got the position data below. X is measured from the centre line outward, and Y is measured from where the hack which is where the rock first touches the ice to where it stops.

Table #1


Table #1 produces the two graphs Figure #1 and Figure #2 which are position plots following the path of the curling stone down the ice.


Figure #1 Figure #2

The position of the rock when before a X is experienced is not shown. Only when a X was experienced did we start to record position data.

Discussion:

Figure#1 and Figure#2 representing the path of the stone down the ice are somewhat inaccurate in that the data should produce a smooth curve. The difficulty in calculating decent position data is perspective. As the camera is set at one end and raised off the ice, distances even 10m away from the camera become inaccurate between +0. . The further the distance from the camera or the closer to the stones initial position the less accurate our results are.
However, even with Figures #1 and #2 unsmooth, we see the basic principle of motion for a curling stone. The slope of the line X vs. Y becomes less and less as the rock travels down the ice, or in other words, as Y increases. This means as the rock approaches its final position, it has a greater displacement in the X direction and less displacement in the Y direction per change in time.

But Why does a curling stone curl?

The rotation of the stone is directly related to the direction in which a stone will curl. As seen above in Figures #1 and #2 a stone rotating in a clockwise direction will curl to the right and a rock that is rotating counterclockwise will curl towards the left.
What does this mean, well this tells that there is a difference in the forces felt between the edge that is rotating in the direction of motion of the stone and the other that is rotating in opposition to the rotation of the stone. See Diagram #2 below.

Diagram #2

In order for the stone in Diagram #2 to curl to the right the opposing side must experiencing a greater force of friction that Favouring side. A curling stone is supposed to make 3 to 4 complete rotations in its total trip. This means that the low rate of rotation has a powerful affect on the nature in which a stone curls.

Factors Affecting Curl

Factors that affect how much a curling stone curls are the curling stone's running surface, the surface of the ice. See Diagram#3 below.


Diagram #3


A flat or unusually 'smooth' surface will tend to curl more towards the last few meters of travel than a stone that has a 'sharper' running surface. It can be explained in terms of the force of friction felt by the different types of running surfaces. The smooth running surface will have less frictional force and will not stop as quickly or will tend to 'slide further' in the last few meters of travel than a sharp stone. This means that the smooth rock will spend a longer time at slow Y velocities which enables a greater X to occur because a curling stone, as seen from above, curls the most per X at slower Y velocities.
The ice surface mainly depends on the way in which the ice is made. When the ice is pebbled, which involves walking down the centre line of the ice evenly spraying small droplets of water in a side to side motion. As rocks pass over this and wear it down or make the surface of the ice flat, then the pebbling is repeated. Because the majority of curling rocks are thrown down the centre, the pebble on the sides of the ice is rarely worn flat. The pebble gets built at the sides of the ice creating a bowl like shape on the surface of the ice. An exaggerated view of this is shown in Diagram #3
Suggestions for futher study that would provide a greater understanding of the physics of curling are:
A. How does the rotation rate affect how much a rock curls?
B. Find the static and kinetic coefficient of friction for ice using
a newton force meter and a curling rock

Acknowledgements:

  1. Dr. Hawkes for partial explanation of the physics of why a stone curls.
2. Sackville Curling Club for the use of the ice.

References:

Maybe not towards the good of the author: NIL

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