# Forces: Atmospheric pressure

## Atmospheric pressure - What a drag!

Different planetary bodies in the Lander game have different atmospheres. The information below explains how this will affect the spaceship and why.

### Objects in motion

The geometry of objects has a considerable effect on the amount of drag they experience. The drag is directly proportional to the area of the object, such that if you double the area, the drag also doubles. The area being considered here is the frontal surface area of the object. For example, if you were stood directly in the path of motion of the object, the area of the object that you can see as it moves towards you is the frontal surface area.

Skin friction also provides a source of drag, with a smooth surface producing less drag than a rough one.

### The motion of the air

Drag is associated with the movement of an object through the air, and so depends upon the relative velocity between the object and the air itself. Drag actually varies with the square of the relative velocity, and so double the relative velocity and you have four times as much drag, triple the velocity and you have nine times as much drag.

Think about how much you feel the air as you’re walking down the road, and then how much you feel when cycling down the road. This is a perfect example of the how the drag increases with the square of the velocity.

A special case occurs when the relative velocity reaches the speed of sound, as shockwaves begin to form on the object, forming an additional source of drag known as “wave drag”.

### The properties of the air

Since the drag is dependent on the mass of the air flowing past the object, atmospheric density plays a factor in the magnitude of drag.

### Drag force equation

All of these factors can be combined into a single drag equation:

**F _{D}=½CρAv^{2}**

In this equation, **D** = Force of Drag, **C** = a constant which varies for different objects (usually determined experimentally), **ρ** = density, **v** = velocity, and **A** = surface area.

As it turns out, we can use different surface areas for A to calculate our drag, such as the total surface area, or the wing surface area. Doing so changes our coefficient K, such that the equation will still produce the same value for drag regardless of which area is used, provided the same area is used consistently to obtain our value of K.