Why does a curling stone, which is following a certain path, sometimes suddenly "hook from this path near the end of its run?
Why does a fast rock barely curl at all, whereas a slow stone sometimes curls an astonishing amount?

Just what ARE the forces that make a rock curl?

First, it is necessary to understand how a stone is able to move on ice.

The pressure of the 40-pound rock melts the ice surface which it comes in contact with, creating a thin layer of water, this water reduces the resistance or friction encountered between the rock and ice and enables the rock to move. The colder the temperature, the harder it will be to get and keep the rock in motion.

The same situation occurs in skating. Your weight exerted on the skate blade melts the ice under the blade - the skate actually rides on this thin film of water. If you have ever been skating in below-zero weather, you will readily appreciate how difficult it is to melt the ice - it then becomes very difficult to move at all, the frictional resistance is so great.

The type of ice surface played upon influences your game considerably. When the ice is pebbled, the stone comes in contact with less ice area than it would if the surface were unpebbled - only a portion of the running surface of the stone are in contact with the ice instead of the entire running surface area. Friction will naturally not be as great. However, as the pebble wears off and the stone gets closer to the ice surface, friction increases.

Possible forces which could cause a rock to curl:

1. Air Pressure

It has been suggested that the unequal flows of air on opposit sides of a moving rock will cause the rock to pull it to one side, according the application of a theory known as Bernoulli's Principal.

This, however, cannot explain why a curling stone curls. Because a curling stone is rotating so slowly, the friction with the air or pileup of the air on one side more than the other, must be negligible.

Dr. Harrington, late professor of physics and an ardent curler, conducted an experiment on an indoor ice rink in order to show the complete inadequacy of the air pressure theory as applied to curling. A number of stones were thrown down the ice and an electic vacuum cleaner was carried abreast of each stone thrown. The air blast from the nozzle was aimed at the stone in the opposite direction to the normal curling direction of the stone. The stone could not be made to depart appreciably from its path, even though the turn imparted was counter the direction of the air blast, the stone curled directly against it - and this blast of air was many times stronger than ever encountered by a stone in a curling game.

2. Ice Friction

The problem now involves studying the ice friction encountered by a moving rock, in order to deduce the causes underlying the motion of curling stones. A moving stone has two components of velocity: (a) a forward speed relative to the ice, which is called linear velocity; and (b) a rotational motion about the center of the stones own gravity, which is called angular velocity. This angular velocity is produced when you apply a torque or twist to your stone in order to give it an in- or an out-turn.

Suppose that a stone is thrown with an in-turn. The right hand side (viewed from the point of delivery) travels with resect to the ice surface at a slower speed than that of the left hand side. The left side of the moving stone has a combined speed of S+s (forward velocity + angular velocity (because the left side rotation is going with the forward motion)), whereas the right side has a combined speed of S-s (forward velocity - angular velocity (because the right side rotation is going away from the forward motion)).

In other words, there is greater friction on the right side of the stone.

Why does the stone sometimes 'hook' near the end of its run?

If the stone has an angular velocity that is relatively high, the velocity of the slow side may actually become zero as the speed of the stone decreases. What has happened is that the rotational speed has reached a point where it is equal to the forward speed. Because the rotational speed is going in a direction away from the direction of the forward speed, the forces cancel themselves out and the right hand side is actually inert, there is no movement at the point of contact between the stone and the ice on the on the right side. The right side 'sticks' to the ice and the stone will seem to whirl about the contact of the slow edge as a pivot. This is the 'hook' that you sometimes observe near the end of a rocks run.

Fast and slow rocks will actually curl the same amount, provided they are allowed to run their full course. A fast rock does not curl because the velocity is always above the critical speed - i.e. the speed at which friction becomes greater and has its effect on the slower moving side.

A fast-spinning rock doesn't grab the ice surface as hard as a slowly curling stone - hence less curl. Watch the rear wheel of a car on ice and you can see the same occurance. If the tire skids, it is spinning around at a high speed relative to the ice - but the tire doesn't grip the ice. When the tire does not skid, it is rvolving slowly at a low speed relative to the ice - and there is more traction. The tire grips the ice and moves in the direction desired.