FORCES AND FLUIDS
8TH GRADE PHYSICAL SCIENCE
FORCES AND FLUIDS
UNIT VOCABULARY LIST
FLUID
ATMOSPHERIC PRESSURE
DRAG FORCE
PERPENDICULAR
SEA LEVEL
PRESSURE
BUOYANT FORCE
SURFACE TENSION
PARALLEL
WHAT IS A FLUID?
A fluid is any substance
that can flow and take the
shape of the container that
holds it.
This means that liquids AND gasses are fluids.
• ATMOSHPERIC PRESSURE the ratio of the weight
of all the air above you to your surface area
• BUOYANT FORCE an upward force applied by
a fluid on an object in the fluid
• DENSITY the mass per unit volume of a
substance
• DRAG FORCE a force that opposes the motion
of an object through a fluid
• FLUID any substance that can flow and take
the shape of the container that holds it
• PARALLEL lines, planes, surfaces, or objects side by
side and having the same distance continuously
between them
• PERPENDICULAR at an angle of 90˚ to a given line,
plane, or surface.
• PRESSURE the amount of force per unit area applied
to an object's surface
• SEA LEVEL the level of the ocean's surface, used as
a reference point when measuring the height or
depth of the earth's surface
• SURFACE TENSION the tightness across the surface
of water that is caused by polar molecules pulling
on each other
FLUID MECHANICS
Fluid Mechanics is the study of forces
that develop when an object moves
through a fluid medium.
A fluid medium can be a gas or a
liquid.
•Two fluids of interest:
1. Water
2. Air
FLUID FORCES
• In some cases, fluid forces have little effect
on an object’s motion (e.g., shot put)
• In other cases, fluid forces are significant –
badminton, baseball, swimming, cycling,
etc.
• Three major fluid forces
1.Buoyancy
2.Drag
3.Lift
DRAG AND LIFT
Drag force acts in a direction that is opposite
of the relative flow velocity.
Affected by cross-section area (from drag)
Affected by surface smoothness (surface
drag)
• Lift force acts in a direction that is
perpendicular to the relative flow.
The lift force is not necessarily vertical.
DRAG
Drag is a resistive force acting on a body moving through
a fluid (air or water).
Two Types of Drag
Surface Drag: Depends mainly on smoothness or surface of
the object moving through the fluid. Examples: Wearing
racing suits in skiing and speed skating
Form Drag: Depends mainly on the cross-sectional area of
the body presented to the fluid. Examples: Bicyclist: in an
upright v. crouched position. Swimmer: related to
buoyancy and how high the body sits in the water.
When would you want to increase drag?
LIFT
Lift represents a net force that acts perpendicular to
the direction of the relative motion of the fluid.
• Created by different pressures on opposite sides of
an object due to fluid flow past the object.
• Example: Airplane wing (hydrofoil)
• Bernoulli’s Principle: velocity is inversely proportional
to pressure.
Fast relative velocity  lower pressure
Slow relative velocity  higher pressure
EXPLAINING LIFT AND DRAG
Drag force: acts in a direction that is opposite of the
relative flow velocity (i.e., it opposes the relative flow)
Affected by surface area (from drag)
Affected by surface smoothness (surface drag)
Lift force: acts in a direction that is perpendicular to
the relative flow.
The lift force is not necessarily vertical.
BUOYANCY
Buoyancy is associated with how well a body floats or how
high it sits in the fluid.
• Archimede’s Principle: any body in a fluid medium will
experience a buoyant force equal to the weight of the
volume of fluid which is displaced.
Example: A boat on a lake.
A portion of the boat is submerged and displaces a given
volume of water. The weight of this displaced water equals
the magnitude of the buoyant force acting on the boat.
The boat will float if its weight in air is less than or equal to the
weight of an equal volume of water.
Buoyancy is closely related to the concept of density.
Density = mass/volume