Fluid resistance is a force exerted by a fluid that opposes an object's motion relative to that fluid. Air resistance is the most familiar example.
The magnitude of fluid resistance generally increases with relative speed and also depends on shape, cross-sectional area, and the fluid. It can therefore make acceleration non-uniform even when gravitational field strength is approximately constant.
For a falling object, increasing speed produces increasing upward resistance, so the downward resultant force and acceleration decrease. A terminal speed is reached when the forces balance.
For a projectile, fluid resistance changes the velocity and acceleration throughout the flight. Compared with an ideal no-resistance model, it can alter the trajectory, time of flight, range, impact speed, and symmetry between ascent and descent.
A.1 requires a qualitative treatment of fluid resistance. Do not assume a particular drag model such as or unless it is supplied. These two expressions mean that resistance is proportional to speed and to speed squared, respectively.
For the A.2 model of viscous drag on a small sphere, the IB Physics guide and data booklet give the magnitude relation
where is the fluid viscosity, is the sphere's radius, and is the sphere's speed relative to the fluid. For fixed and , this particular model gives . It is not a general drag law for every object or flow.
The drag force direction is opposite to the object's velocity relative to the fluid. Because that direction changes for a projectile, fluid resistance cannot in general be represented by one constant acceleration.