P2aL7 Moving through Fluids

Key Words

Air resistance - drag experienced by moving through air.

Density - mass per unit volume, or how heavy something is for a given volume.

Drag - resistance to movement through a fluid.

Friction - force opposing movement caused by surface rubbing against each other.

Gravity - an attractive force that pulls objects to the Earth.

Speed - distance covered in a period of time.

Streamlined - long thin shape that allows an object to pass through a fluid with the least possible drag.

Terminal velocity - the maximum downwards velocity of an object falling through a fluid.

Upthrust - upwards force that results from the action of a fluid.  It opposes the weight.

Weight - a force acting on an object caused by gravity.

Grade E

Air is a fluid.  A fluid is any liquid or gas; it can flow around objects.  Drag arises because:

• particles of the fluid have to be pushed out of the way;

• particles rub along the object.

Both of these contribute to the phenomenon of drag.

As the the speed gets higher, the drag gets bigger because more particles need to be pushed out of the way every second, and the friction of them rubbing against the sides get bigger too.  Also some fluids have higher densities.

As before, all objects will reach a terminal velocity in a fluid.  In air a human will fall at 60 m/s; in water the terminal velocity would be about 0.6 m/s.

A simple experiment to show this is to drop a ball bearing through oil.  The more viscous (gooey) the oil, the lower the terminal velocity.

Grade C

Water makes much more drag than air, so boats tend to be much slower than say cars or even bicycles.  A power boat manages to get to a high speed because it is designed to lift out of the water and to plane or skim along the surface of the water.  Therefore there is only a little drag from the water, and air resistance.

An eight oared racing shell is about 18 metres long but only 50 cm wide.  It is very streamlined and can cut through the water at quite high speed, about 6 m/s.

Other fasted moving objects are streamlined.

Turbulent water has less upthrust because there are bubbles of air, making the density of the water a lot lower.

Grade A

As well as the density of the material viscosity is important.  Some fluids have particles that are tightly bonded, so need more work to push them apart.  This is true of many oils, which are less dense than water.