The table below provides examples from balloon cars, 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 balloon cars to work, sometimes it prevents the balloon cars from working.
- Wheels fall off:: In the Troubleshooting section of Lesson 8, “Things fall apart”, we suggest different ways to keep wheels on. Students sometimes invent their own ways, too. The important thing is that the wheels remain able to turn freely on the axle.
- Wheels fixed to axle and axle fixed to cardboard: nothing moves: In the Troubleshooting section of Lesson 8, we describe how to allow the axle to turn easily with respect to the cardboard, to which it is fastened: place the axle through a straw “bearing” and then fasten the straw to the cardboard.
- The force of the air from the balloon is in the wrong direction: nothing moves: See the Troubleshooting section, “Getting the car to go”, in Lesson 8. When the air from the balloon pushes up, it tries to push the car down and it is stopped from going down by the floor. When the air pushes down it tries to make the car go up, but the weight of the car is too great for it to overcome gravity. When the air from the balloon pushes to the side, the only way for the car to move is for the wheels to slide sideways across the floor. The sliding friction between wheels and floor is too great for the car to move. The car won’t go unless the airflow pushes it in the direction where the wheels can roll.
- The weight of the car is too great: nothing moves: As the car weight increases, the friction between axle and straw, wheel and axle, and wheel and floor increase. There comes a point where the forces from friction are greater than the force from the air blowing out the balloon. When that happens, the car won’t move.
As the balloon car is rolling, there is some friction between the wheels and the floor, between the axle and wheels or the axle and the straw bearing and between the whole balloon car and the surrounding air. All of this rubbing causes everything to heat up ever so slightly. The heat energy gained is equal to the kinetic energy that is lost. 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.