What causes the drag effect in fluids?
What is air resistance? What has it got to do with friction?
You first appreciate friction when two solid
surfaces rub together. When you pull a heavy object across the floor you
experience the resistive force between the two surfaces. This resistive force is
called friction and is caused by the atoms of the two surfaces bumping in to
each other from opposite directions. The moment you stop applying a force, the
dragged object ceases moving immediately.
The force of friction always acts in the
opposite direction to an object's movement and it can involve ANY type of
surface contact effect.
Therefore when a solid object moves
through a fluid - gas or liquid, there are collisions between the fluid
particles and the surface of the object which creates the resistive force of
friction.
In fluids this resistive force is called
the drag effect. A boat
ploughing through water experience the drag effect of the water brushing
over the surface of the hull. An aircraft of skydiver moving through the
atmosphere experience a drag effect, which in this case is called
air resistance.
If an object is moving at a steady speed the
thrust or driving force (engine or gravity etc.) is being balanced by the
opposing force of friction.
e.g. a car moving at a constant velocity
means the thrust force from the engine is balanced by the friction of
moving parts in the engine, wheel contact with the road surface and the air
resistance (the latter being the drag effect).
If you increase the power output of the
car by pressing harder on the accelerator pedal you overcome more of the
resistive force and move faster, but the friction e.g. the air resistance
increases with speed (see next section), so eventually all the opposing
forces balance and you attain a higher steady speed.
BUT, if you want a faster constant
speed, the engine must work harder and burn more fuel to overcome the
increased drag effect as air resistance increases with speed.
The drag effect is due to air
resistance, that is the air brushing over the surface of the moving car.
If you take your foot of the accelerator
and apply the brake, you increase the force of friction so the car slows
down. Theoretically the drag force is reduced with decreased speed, but its
a very small effect here compared to the friction between the brake pads and
brake discs on the car wheels.
If a moving object has no thrust force acting
on it, then it will always slow down and come to a halt e.g. on a level road,
if you take your foot of the accelerator, the car will eventually come to a halt
as the resistive forces act against the car's forward motion. The total friction
effects will bring the car to a halt.
An object can keep moving if it is
subjected to a constant force greater than the friction effects e.g. a car
will roll down a hill due to gravity.
You should also appreciate that objects
will move forever in outer space because there are virtually no molecules to
bump into and cause a friction-drag effect.
Ships moving through water experience a drag effect due to
the boat hull surface experiencing friction as it moves through the water.
