Forces & Motion

Air Resistance

The force that resists the motion of an object through a gas and liquid is called drag.

For objects moving through air sometimes instead of drag the term air resistance is used. As an object moves through air, the gas molecules in the air push against the surface of the moving object resulting in friction between the gas molecules in the air and the surface of the moving object.

The amount of drag encountered depends on the following:

Shape

An object with smooth lines will allow the air to flow over it more easily reducing the drag. This is known as streamlining and is why objects built for speed such as racing cars have smooth bodies.

Speed

Drag increases with speed.

Area

The larger the area of contact the more drag it will experience.

The type of fluid

There will be more drag in a liquid compared to a gas as the molecules are a lot closer together. A stone will fall much faster in air than in water.

Terminal Velocity

When an object falls it accelerates due to its weight (the downward force of gravity acting on the objects mass). As it accelerates its velocity increases. The increase in velocity is accompanied by an increase in air resistance (drag). Eventually the air resistance acting upwards on the objects equals the weight acting downwards. The overall force on the object is balance or zero; it therefore cannot accelerate and continues to fall at constant velocity. This is referred to as the terminal velocity.

The animation below shows the terminal velocity of a skydiver: