Friction is a force that opposes movement between two surfaces
Drag is a frictional force that slows things down in air and water (gases and liquids)
Streamlined shapes move more easily through air and water (gases and liquids)
EXAMPLE QUESTIONS
How can you make a slippery surface?
How can you stop things slipping?
How could you make things ‘slow down’ or ‘speed up’?
FRICTION
Friction is the name we give to the force that slows down moving objects.
It always acts in the direction opposite to that in which the object is moving.
When friction is high, surfaces cannot slide easily over one another.
This can be helpful when a good grip or slowing down is necessary.
Friction can be reduced by making surfaces slippery.
When friction is low, surfaces can slide easily over one another. For example, snow on the hillside allows the skis to slide easily because the force of friction is reduced.
It would be difficult to ski down a hillside without snow; the skis rubbing against the surface would be stopped by the force of friction.
In some circumstances, low friction can be a nuisance or dangerous, as when driving on icy roads.
DRAG
Drag is the name given to a particular kind of frictional force that acts on objects when they move through a liquid or gas.
The object has to push aside the liquid or gas to get through, so the liquid or gas pushes back on the object.
Sometimes it can be useful because it slows down moving objects (such as air pushing up on a descending parachute).
Objects can move faster through air and water (and other gases and liquids) if their shapes are streamlined to cause as little drag as possible.
Streamlining helps the object ‘cut through’ gases or liquids.
Cars, aeroplanes and rockets are streamlined for this reason.
