The Physics of Ice Skating - Torque

       Torque is a rotational force, in fact the word itself comes from the latin for “to twist”. Torque, in a sense, causes rotation about an axis. Torque involves both the force applied to an object and the distance from the rotation axis you apply the force. Perhaps some examples will help. In order to open a heavy door, you need to apply a force. But force alone will not do the job. Where the force is applied and in what direction is also important. If you apply a force close to the hinge of the door rather than out by the doorknob, it is much harder to move the door. That’s why doorknobs are located at the opposite side of the door to the hinges; it’s much easier to move the door out there.

       The definition of torque is the product of the distance from the axis of rotation (often called the lever arm) with the force that is perpendicular to the lever arm. (If you pull on a door parallel to the plane of the door, you do not rotate the door.) Another example of torque occurs when you turn a screw or bolt. Using a screwdriver (the non-electric kind) is often hard and time consuming since you must apply a large force in order to turn the screw (small lever arm). However, if you use a wrench for tightening bolts, you only need to apply a small force since you have a long lever arm. That's why wrenches used to turn large bolts have much longer handles compared to those that turn small bolts. This enables the user to use less force since they have a long lever arm. Of course, the user must apply that force over a longer distance.  So there's a tradeoff between force and distance.

       So how do we get from tools to ice skating? A skater, in order to rotate, must exert a torque on his body by pushing against the ice. In edge spins, the skater pushes one foot against the ice to start the turn. You also see this in multiple rotation edge jumps. In these jumps, the skater takes off from the ice, turning the skate as he does so, which creates a torque.  Thus, the skater spins!

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by Karen Knierman and Jane Rigby