# Friction and the Wind-up

The table below provides examples from wind-ups, showing the many faces of friction. In each case there are two surfaces in contact, between which there is friction. Sometimes the friction is necessary for the wind-up to work, sometimes it prevents the wind-up from working.

The bead is used to reduce friction: In the Troubleshooting section of Lesson 2, we explained that the bead is needed between the stick and the wheel to reduce the friction between them. If there stick is too tight against the wheel, there will be too much friction and the stick and wheel will not be able to move independently.

• A longer or weaker rubber band reduces friction: : In the Troubleshooting section of Lesson 2, we explained that the friction between the stick, bead and lid can be reduced by reducing the force from the rubber band that holds them together. This can be done by using a longer rubber band or a thinner one.
• The paper clip and lid require friction to keep them together An example – where friction is a good thing – is the attachment of the paper clip to the other wheel. Here they need to move together, or the turning paper clip won’t make the car go.
• Some friction is needed between the wheel and the floor. If the wheels are small, they will slip on the floor, and the wind-up will spin in place rather than travel forward. See Lesson 6. Like a car wheel, something rough is needed to make the wheel “grab the road.” A car uses a rubber tire to increase the friction between the wheel and the road. Of course, too much friction will not work, which is why wheels should not be square.
• As the wind-up is rolling, there is some friction between the wheels and the floor, between the whole wind-up and the surrounding air, and also between the stick and the bead. All of this rubbing causes everything to heat up ever so slightly – probably much less than you can notice – but accounting for all the energy the was previously kinetic. Energy “lost to friction” is not actually being destroyed – it is being converted from mechanical energy to heat energy, which is “lost” only in the sense that it can no longer be used.