Everyone is familiar with gears in human engineered devices. A gear is a device for controlling an output force or displacement or speed given a certain input force. In a bicycle, we apply an input force to the pedal and crank and get an output force of the rear tire pushing the ground rearward, causing the bike to move forward. In a geared bike, a low gear is used for generating large output force – for large accelerations or for climbing a steep hill. A high gear is used at high speed on flat or descending road. This high speed is achieved because the rear wheel is turning many times for each rotation of the crank. This turning is the output displacement. High gear results in high output displacement. The more displacement per unit time (say the time it takes to rotate the crank once), the higher the speed. There is a trade-off in gears – a low gear generates high highput force but low displacement. A high gear results in high displacement but low output force.