Physics Narrative for 11-14
A drag force
When objects move through a fluid (a liquid or a gas) their progress is opposed by a force which acts on the object and in the opposite direction to the movement. We call this a drag force and it acts just like a slip force. A drag force is not so much a rough, rubbing effect as a brushing-by effect. Air resistance is a drag force. Swimmers and high-divers experience a drag force when they try to move through water. Drag arises from wave making and the viscous movement of the particles of the fluid around the moving object. The mechanisms are complex: but identifying the force is not. If an object is moving through one or more fluids then there will be a drag force.
An interesting feature of drag forces is that they become greater as the speed of movement increases. Drag forces can therefore influence motion, either reducing the speed of an object in the case of a bullet or preventing an object from speeding up further in the case of a sky diver.
forces spectacles a drag force is shown by drawing in a force arrow, with its tip at the surface where the fluid has the greatest effect, to remind you of the origins of the force on the object.