KAHIBARO
Table of Contents
Motion Through a Fluid
When an object moves through air, water, or any other fluid, the fluid resists the motion. This resisting force is called drag force. Drag always acts opposite to the relative motion between the object and the fluid.
If a ball falls through air, the drag force points upward because the ball moves downward relative to the air. If a car moves forward through still air, the drag force points backward. If the air itself moves, what matters is the motion of the object relative to the air, not relative to the ground.
The drag force always opposes the relative velocity between an object and the fluid.
Why Drag Appears
A moving object pushes fluid particles out of the way. At the same time, the fluid rubs against the surface of the object. These effects create a force that resists motion. In simple terms, drag comes from two main causes, pressure differences around the object and friction with the fluid.
A streamlined object, such as an airplane wing or a fish, is shaped to reduce drag. A blunt object, such as a flat board facing the flow, produces much larger drag.
Direction of the Drag Force
The direction of drag is simple but very important. It is always opposite the velocity relative to the fluid. If the object reverses direction, the drag force reverses too.
For one dimensional motion, people often write drag using a minus sign to show opposition. For example, if motion is along the $x$ axis, one possible model is
$$F_d = -bv$$
or
$$F_d = -cv^2$$
where the minus sign means the force points opposite the motion.
Common Mathematical Models
There is no single drag formula that works in every situation. Two common approximations are used for beginners.
For low speeds or very small objects moving smoothly through a fluid, drag is often approximately proportional to speed:
$$F_d = bv$$
where $b$ is a constant that depends on the object and the fluid.
For higher speeds in air or water, drag is often approximately proportional to the square of the speed:
$$F_d = cv^2$$
where $c$ is another constant.
In vector form, the force is opposite the velocity direction. A compact way to write quadratic drag is
$$\vec{F}_d = -c v \vec{v}$$
because $v = |\vec{v}|$, so the force points opposite $\vec{v}$.
Two important drag models are
$$F_d = bv$$
and
$$F_d = cv^2$$
The actual force direction is opposite the motion, so signs or vector notation must show that opposition.
Dependence on Shape, Size, and Fluid
Drag depends on more than speed. It also depends on the shape and size of the object, and on the properties of the fluid.
A larger front area usually gives more drag. A rough or irregular shape often gives more drag than a smooth streamlined one. Denser fluids, like water, usually produce much more drag than air.
A commonly used expression for quadratic drag is
$$F_d = \frac{1}{2} C_d \rho A v^2$$
where $C_d$ is the drag coefficient, $\rho$ is the fluid density, $A$ is the cross sectional area, and $v$ is the speed relative to the fluid.
This formula is very useful because it shows the main physical factors clearly.
| Symbol | Meaning | Effect on drag |
|---|---|---|
| $C_d$ | Drag coefficient | Depends on shape and surface |
| $\rho$ | Fluid density | Denser fluid gives larger drag |
| $A$ | Cross sectional area | Larger area gives larger drag |
| $v$ | Relative speed | Higher speed gives much larger drag |
A widely used drag formula is
$$F_d = \frac{1}{2} C_d \rho A v^2$$
This is especially common for objects moving through air at ordinary speeds.
Drag in Falling Motion
A very important example is a falling object. Two main forces act on it, weight downward and drag upward. At first, when the object starts from rest, drag is small because speed is small. As the object speeds up, drag grows. Eventually drag can become large enough to balance the weight.
When that happens, the net force becomes zero, so the acceleration becomes zero. The object then continues falling at a constant speed called terminal velocity.
For a falling object with quadratic drag, terminal velocity occurs when
$$mg = \frac{1}{2} C_d \rho A v_t^2$$
so
$$v_t = \sqrt{\frac{2mg}{C_d \rho A}}$$
A heavier object tends to have a larger terminal velocity if other factors stay the same. A larger area or larger drag coefficient lowers the terminal velocity.
Terminal velocity is reached when drag balances weight.
At terminal velocity,
$$F_{\text{net}} = 0$$
so the object moves at constant speed.
Comparison With Friction
Drag force is similar to friction because both oppose motion, but they are not the same. Friction usually acts between solid surfaces in contact. Drag acts when an object moves through a fluid.
Another difference is that drag usually depends strongly on speed, while simple kinetic friction is often modeled as nearly independent of speed.
| Force | Medium | Opposes | Typical speed dependence |
|---|---|---|---|
| Friction | Solid surfaces | Relative sliding | Often weak |
| Drag | Fluid | Relative motion through fluid | Often proportional to $v$ or $v^2$ |
Simple Physical Picture
Imagine moving your hand slowly through water. The water resists a little. Move your hand much faster, and the resistance becomes much stronger. This is the basic behavior of drag. At low speed the increase may be roughly linear, and at higher speed the increase is often closer to quadratic.
This is why cyclists crouch down, skydivers spread or tuck their bodies, and cars are designed with smooth shapes.
Visualizing Drag on a Moving Object
Falling Object and Terminal Velocity
Key Ideas to Remember
Drag force is the force a fluid exerts to oppose motion through it. Its direction is opposite the relative velocity. In simple models, drag is proportional to $v$ at low speed and proportional to $v^2$ at higher speed. Drag depends on speed, shape, area, and fluid density. In falling motion, drag can grow until it balances weight, producing terminal velocity.
Essential facts about drag force:
$$\text{Direction of drag} = \text{opposite to relative motion}$$
Common models:
$$F_d = bv$$
$$F_d = cv^2$$
Useful air resistance form:
$$F_d = \frac{1}{2} C_d \rho A v^2$$
At terminal velocity:
$$F_d = mg$$