IP2.7.5 Air resistance © Oxford University Press 2011 Air resistance

IP2.7.5 Air resistance © Oxford University Press 2011  Friction is all around us. If you push the palm of your hand along a desk you feel the effects of friction.  If you push your hand gently you can feel the frictional force between your hand and the table – it feels like your hand is glued to the table and it doesn’t move (this is static friction). It is what helps cars to stick to the road when accelerating.  If you push your hand harder it slips. If you keep moving your hand around quickly on the desk you will feel your hand get warm (this is kinetic friction). Kinetic energy is changed into heat. This is what happens when a car skids.  These are all examples of friction between two solids. There is also a sort of friction when a solid moves through a fluid. It is called fluid resistance or drag.

IP2.7.5 Air resistance © Oxford University Press 2011  Think of the effort it takes to stir a spoon in some honey.  This is an example of the fluid resistance that objects experience moving through a liquid.  Air is also a fluid (although it is a gas and thinner than honey). As objects move through air they experience a resistive force against the direction of motion.  Think of the car having to push the air out of the way.  Air resistance is the largest force opposing a car’s motion.

IP2.7.5 Air resistance © Oxford University Press 2011  Some of the factors that affect the size of the resistive force of air resistance are:  The speed of the object – as the speed increases the air resistance increases.  The surface area of the object – the larger the surface area the higher the air resistance.  The shape of the object – blunt/flat shapes experience a higher air resistance than streamlined pointy shapes.