HOW FAST CAN IT GO?

As the rubber band unwinds, it tries to turn the popsicle stick and the pencil. The friction between the popsicle stick and the can is too great to move the stick. The pencil can move because the bead and plastic disk have less friction between them. However, when the pencil tries to turn, it pushes on the floor (which pushes back) and so the only way the rubber band can unwind is to roll the can forward. As the can rolls, the potential energy that is stored in the wound rubber band is transformed into motion (kinetic energy) and heat. The can will roll if the energy stored in the wound rubber band produces a force great enough to overcome the force of friction between the bead and the disk and between the can and the floor.

### Objectives

• Build a compound machine that converts potential energy to kinetic energy.

### Materials

• Popcan
Hammer
Nail
Rubberbands
Pencil
Plastic disk (cut from an ice cream bucket or lid)
Wire (bent coathanger)
Popsicle stick

### Key Questions

• What is causing the can to roll? What is potential energy? What is kinetic energy?

### What To Do

1. Using a hammer and nail, make a hole in the bottom of a pop can. The hole must be big enough for a rubber band to pass through.
2. Cut a plastic disk (from an ice cream bucket or lid) to fit the top of a pop can. Poke a hole through the centre of this disk.
3. Thread the rubber band through a plastic bead then through the centre of the plastic disk. Loop it over a pencil so it does not pull back through the bead.
4. Using a long wire hook, pull the rubber band through the top of the pop can and out the bottom. Loop this end of the rubber band over a popsicle stick so that it cannot be pulled back into the can. (The popsicle stick must be broken so that its length is less than the diameter of the can).
5. Wind the pencil around and around until the rubber band is tight. Set the Pop Can Porsche on the floor so that as the pencil tries to unwind it pushes on the floor.
6. Release and watch your Pop Can Porsche roll away. If it slips and spins on the floor, wrap rubber bands outside the can to act as tires.

### Extensions

• Design an experiment that increases the challenge: introduce a ramp or reduce the friction between the can and the floor.

### Other Resources

Science World at TELUS World of Science | School Programs |  Simple Machines
Science World at TELUS World of Science | Eureka! Gallery