Magic spool

HOW

1. Place the spool in the middle of the base length.
2. Unroll about a metre of tape from it.
3. The point where the tape loses contact with the spool should be at the bottom of the spool.
4. Gently pull the tape. Hold it so that it forms a small angle with the level. What can you see? Which way does the spool roll?
5. Now try pulling the tape gently, holding it so that it forms a large angle with the level. Which way does the spool roll now?
6. Try to make the spool slide without rolling.

WHY

The movement of the spool consists not only of its translational motion, but also of its rotational motion. Your impact on the spool tape is equivalent to exerting some force on the spool itself. The distance of the point of force application from the axis of rotation is known as the lever arm.

The instantaneous axis of rotation is at the point where the spool makes contact with the ground. In rotational motion, the so-called torque is created. This means that the direction of its vector coincides with the spool’s axis of rotation, and the sense depends on the mutual alignment of the applied force and the arm of that force.

What is happening?

• If the axis of rotation of the spool (point of contact with the ground) is exactly at the extension of the part of the tape behind which the spool is pulled, the torque does not act. As a result, the spool slides without rotation (see Figure b).

• If the angle between the track of the spool and the stretched tape is greater than in the case discussed earlier, the torque causes the spool to move away (Figure a).

• When this angle is smaller, the torque causes the spool to approach (Figure c).