FORCES AND FLUIDS

8TH GRADE PHYSICAL SCIENCE

FORCES AND FLUIDSUNIT VOCABULARY LIST

FLUID PRESSUREATMOSPHERIC PRESSUREBUOYANT FORCEDRAG FORCE SURFACE TENSIONPERPENDICULAR PARALLELSEA LEVEL

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 MECHANICSFluid 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. Water2. 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 forces1.Buoyancy2.Drag3.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.

DRAGDrag is a resistive force acting on a body moving through a fluid (air or water). Two Types of DragSurface Drag: Depends mainly on smoothness or surface of the object moving through the fluid. Examples: Wearing racing suits in skiing and speed skatingForm 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 pressureSlow 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.

BUOYANCYBuoyancy 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