What is terminal velocity? (falling bodies)

When an object falls down through a fluid (e.g. water, oil, air..) there are usually two forces acting on it. They are the weight force acting downwards and the drag force acting upwards. The weight force comes from the gravitational attraction between the object and the earth, and the drag force is caused by collisions between the object and the particles that make up the fluid- as the objects falls it bumps into the particles of the liquid. When the weight force and the drag force balance, there is no resultant force acting on the object. With no resultant force acting on the object, its velocity will remain constant, and this is what is known as its terminal velocity. Lets do an example. Imagine a skydiver jumping out of a plane, so here the object is the skydiver and the fluid is the air. Just as the skydiver jumps out of the plane, his downward velocity is roughly zero. The drag force has a velocity dependence- the higher the velocity, the more often the skydiver bumps into the air particles, the higher the drag force. Therefore, when the skydiver's velocity is zero, the drag force is zero, so the only force acting on the skydiver is his weight, so the resultant force is downwards, and he accerlates downwards. `As he accelerates and his velocity increases, the drag force acting on him increases. This happens until the drag force and the weight force balance, and the skydiver reaches their terminal velocity.