Does the shape of the car affect its speed?

Answer 1

The shape of the car (or plane or boat) has a very important consequence on the resistance to motion. It is a subject in aerodynamics (if it is air) or hydrodynamics (if it is water) or some people just call it fluid dynamics. The discussion here for air is pretty much the same as for other fluids.

When a car or any object goes through air there is air resistance. Air resistance is experienced as a force that pushes against the motion forward. It is a force that increases as the speed of the object increases. Higher speed means higher force and more energy expended to maintain the speed.

What follows is a brief discussion of why the shape of the object matters.

The air must be push out of the space ahead of the object and since the air has mass, it experiences and acceleration and that requires force between the air and the moving object. There are more forces between the air and the objects when the air returns to the spaces as the object moves forward. If this is all accomplished without stirring up the air too much, it is called streamline flow and otherwise it is called turbulent flow. Creating turbulent flow takes much more energy and involves a much greater air resistance.

In addition to the effect of pushing the air out of the path of the object, there is also the consequence of the viscosity of air. When any fluid flows, there is some energy required to maintain the flow to overcome the internal resistance to flow called viscosity. That takes energy. Basically, viscosity arises because of the forces between molecules in the fluid. Trying to move some molecules in one region is a certain directions causes them to "bump" into molecules in adjacent regions and lose some of their forward motion. That same kind of transfer of energy takes place between the surface of the moving object and the fluid around it. As a result, that motion is transferred from one region to another and that takes energy. In the case of turbulent flow, it is a lot more energy.

Shape is critical in terms of reducing air resistance. Shape determines how much air has to be moved out of the way and how aggressively that movement has to occur. That translates into how much force and energy is required to keep an object moving.

Shape has an important effect then on both the speed which an object can attain. Shape has an important effect of the energy required and for a car that effects efficiency and gas mileage. Both of the effects are greater at greater speeds.