Why does a curling stone curl?

Answer 1

The Heat makes your hair contract, basically your hair shrinks in the heat in areas pulling the rest of your hair causing it to curl, same thing happens with plastic. People also want it to curl their hair, that's why they invented it.

Answer 2

This question's been debated for a long time. In fact, I remember reading that curling rocks violated the laws of physics, but people have since figured out how it works. I'll explain it to the best of my ability, although I'm sure a search online would provide a more scientific explanation:

The ice on a curling sheet is "pebbled", meaning it's sprayed with a coating of water droplets which freeze and give the ice a coarse, pebbly surface. This is important, since rocks wouldn't be able to curl without the pebble. As a rock is released, it's spun in one direction or another. As the rock spins, instead of continuing on a straight trajectory down the ice, the surface of the rock which is in contact with the ice is constantly spinning and moving across it. Since the rock is sitting on top of a fine layer of pebbled ice droplets, as it spins it's constantly "catching" onto adjacents pebbles, sort of climbing the rock in one direction as it keeps sticking itself to the next pebble over.

Imagine you'd given the rock a clockwise spin as you released it, the movement of the rock along the ice would make the rock move along the ice in a rightward direction. It's hard to explain, but if you visualize how the bottom of the rock is touching the ice surface as it spins, it's easier to understand.

Of course, the purpose of sweeping is to control both the speed and the amount of lateral movement of the rock - by sweeping, the brush melts the ice directly in front of the rock's path, meaning the surface is more slick and there's less "grip" for the rock to move laterally across.

Hope that helped!

Here's another explanation:

The reason a rock curls is because, as it travels down the ice, there is more friction on the trailing side of the running surface than on the leading side. Imagine a rock rotating clockwise as it moves away from you down the sheet. As it rotates then, the leading edge is moving to the "right" from your vantage point, and the trailing edge is moving to the "left." Frictional force acts in the opposite direction of movement, so friction acts to the left on the leading edge, and to the right on the trailing edge. You'd think these forces would balance each other out, but the friction on the trailing edge is greater, so the net force is to the right, and a clockwise-spinning rock "curls" to the right.

Now why is there more friction on the trailing edge? That's because there's more pressure on the leading edge and less on the trailing edge. But wait, that seems opposite! If there's less pressure on the trailing edge, shouldn't there be less friction? Well a neat characteristic about ice is, when you put pressure on it, it melts into a film of water (this is what makes ice skating work). So the greater force on the leading edge of the rock creates a film of water under that edge that decreases the friction, so the trailing edge ends up with greater friction.

Finally, why is there more pressure on the leading edge in the first place? Imagine a rock traveling to the left through your field of vision. As the rock travels left, friction with the ice acts in the opposite direction. If you think about how the rock would act if it were just hanging in free space, able to rotate in three dimensions, that force to the right on the bottom surface on the rock would cause it to tip to the left (remember you're looking at the rock from the side here). So the friction on the bottom of the rock is sort of trying to "tip" the rock forward into the ice (technically speaking, it's creating a torque). But of course the rock can't tip, it's flat on the ice, so there must be an opposing torque at work. That opposing torque comes from the ice pushing up on the rock from the left of the center of gravity (or forward of the center of gravity, however you're thinking about it in your mind now).

So, the ice "pushes up" on the rock at some point ahead of the center of gravity in order to oppose friction. This creates more film at the front, which leads to more friction at the back, which pulls the rock in the direction the rock rotates. Phew.