In practice, elevators work in a slightly different way from simple hoists. The elevator car is balanced by a heavy counterweight that weighs roughly the same amount as the car when it's loaded half-full (in other words, the weight of the car itself plus 40–50 percent of the total weight it can carry). When the elevator goes up, the counterweight goes down—and vice-versa, which helps us in four ways:
- The counterweight makes it easier for the motor to raise and lower the car—just as sitting on a see-saw makes it much easier to lift someone's weight compared to lifting them in your arms. Thanks to the counterweight, the motor needs to use much less force to move the car either up or down. Assuming the car and its contents weigh more than the counterweight, all the motor has to lift is the difference in weight between the two and supply a bit of extra force to overcome friction in the pulleys and so on.
- Since less force is involved, there's less strain on the cables—which makes the elevator a little bit safer.
- The counterweight reduces the amount of energy the motor needs to use. This is obvious to anyone who's ever sat on a see-saw: assuming the see-saw is properly balanced, you can bob up and down any number of times without ever really getting tired—quite different from lifting someone in your arms, which tires you very quickly. This point also follows from the first one: if the motor is using less force to move the car the same distance, it's doing less work against the force of gravity.
- The counterweight reduces the amount of braking the elevator needs to use. Imagine if there were no counterweight: a heavily loaded elevator car would be really hard to pull upwards but, on the return journey, would tend to race to the ground all by itself if there weren't some sort of sturdy brake to stop it. The counterweight makes it much easier to control the elevator car.
In a different design, known as a duplex counterweightless elevator, two cars are connected to opposite ends of the same cable and effectively balance each other, doing away with the need for a counterweight.
