Dr. Cooper designs each wheelchair to fit the person who will be using it and the activities it will be needed for. One way he and his team can change the design of a wheelchair is by changing the mass of the chair. Mass is the amount of matter that makes up an object—on Earth, objects with more mass are heavier than objects with less mass. By changing the mass of a wheelchair, Dr. Cooper can make it change velocity more easily or less easily. That makes each chair useful for certain activities. For example, some wheelchairs are built especially for playing different sports.
Wheelchairs used for wheelchair racing are built for speed. That means they need to be light. Lighter chairs change velocity more easily than heavier ones, so it’s easier for a racer to go from sitting still to racing speed in a light chair. It also takes less force from the racer to stop a light chair than it takes to stop a heavy chair. Dr. Cooper’s racing wheelchairs are made of materials that are light and strong, so the racer can start the race and speed up as quickly as possible using the least possible force
Not all athletes in wheelchairs want to change velocity easily. Another popular sport that uses wheelchairs is wheelchair rugby, a fast, full-contact sport played on a court similar to a basketball court. Wheelchair rugby players need stability—they crash into each other often, and it’s important that they don’t tip over in a collision. For this reason, rugby wheelchairs are heavier than racing wheelchairs. Their weight means players need to use more force to get them moving when they’re stopped and to make them stop moving once they get going, but it also means they aren’t affected as much by the forces involved in collisions, so they are less likely to fall over during a crash. Rugby players in heavy, stable wheelchairs are more likely to stay upright and play successfully for their teams
