The Physics of Ice Skating - Friction

What is friction?  How do skaters switch between low and high friction contact with the ice?

What is Friction?

       Friction is a force that dissipates energy. It occurs when sliding an object over a surface. The molecules in the surface bond with the molecules in the body causing the sliding motion to be resisted. This force is directed along the surface and opposite to the direction of the motion.

       So why do we need friction? It’s friction that enables us to walk, among other things. Friction is what keeps one foot planted on the ground as the other one steps forward. Otherwise, without friction, the shift in weight would cause us to slide around. Also most braking systems use friction as their main component. When you squeeze the brakes on a 10 speed bike, a small rubber brake is pressed against both sides of the bike tire. The friction of the brake pad against the wheel causes the rotational energy of the wheel to be dissipated or removed by transfering it to heat and sound energy.

How do skaters switch between low and high friction contact with the ice?

       Before we can talk about friction in ice skating, we have to know how ice skates are put together. There are two main parts to an ice skate, the boot and the blade. The boot is where you put your foot (the part you lace up.) The blade is what you skate on. It has three features: the rock and the two edges. Ice skates are not constructed to be flat across the bottom. They have a slight curve to them which is put in when the blades are sharpened. This curve is called the rock of the blade. The blade itself is not straight across horizontially along the part you skate on. It is sharpened in a concave (inward curving) curve. This gives the skater two edges to skate upon. The edge toward the instep of both feet are called “inside edges.” The other edges toward the outside of both feet are called “outside edges.” When you skate on both edges, that's called skating on “flats.”

       In ice skating, friction is used to start a stroke. As you angle your foot outward and extend the knee, the inside edge of the blade encounters the ice and the friction between the ice and the blade enables you to encounter resistance as you scrape the blade across the ice. You exert a force from the edge into the ice. Friction enables the edge to grip into the ice. The equal and opposite force of the ice onto the edge acts to propel the skater forward with kinetic energy.

       Friction is invaluable in stopping on ice skates. In order to stop properly, you apply pressure from your edge down into the ice (stopping by running into the boards does not count here). The continuous force from the edge down to the ice creates a drag which slowly removes the kinetic energy of skating across the ice converting it into heat and sound (that familiar scrape of the blades across the ice).

     To see how important friction is in ice skating, try to imagine skating without it!

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by Karen Knierman and Jane Rigby