2.14

Glossary

You will see this glossary at the end of each week. We compiled it from comments and enquiries from previous learners in this course. If you are not clear on a key term or concept this is a good place to start.

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

A

Acceleration

In a straight line it is the rate of change of speed - a scalar quantity. In other situations it is a vector quantity that is much more complicated.

Area

The extent of a surface measured in m2.

B

Bollard

An object for tying boats etc. to.

Bolt

A threaded component with a head in which there is an unthreaded section (a shank) near the head. It is generally used with a nut.

Built in end

A connection between two bodies that can restrain translation and rotation.

C

Calculus

A wonderfully elegant mathematical technique that we won’t be using in this course.

Cantilever beam

A beam that is built in to a wall.

Centre of gravity

The point on a body where all the weight may be considered to act.

Centroid

A geometric property of a shape. If the shape is made of material of uniform density it is the same place as the centre of gravity (for most practical purposes).

Coefficient of drag

A description of the aerodynamic drag force of an object of a particular shape in a fluid flow. , where Drag = 0.5 * $p$ * A * CD * v2 where A = projected area, $p$ = fluid density, CD = drag coefficient and v = fluid velocity

Coefficient of kinetic friction

The friction force on a moving object on a rough surface, as used in the equation Friction force = (normal force) * (coefficient of kinetic friction).

Coefficient of lift

A description of the aerodynamic lift force of an object of a particular shape in a fluid flow, where Lift = 0.5 * $p$ * A * CL * v2 where A = projected area, $p$ = fluid density, CL = lift coefficient and v = fluid velocity

Coefficient of limiting static friction

The maximum friction force that an object on a rough surface can sustain before it starts to slide, as used in the equation Friction force = (normal force) * (coefficient of limiting friction).

Component

One of a number of forces that between them can replace a resultant.

Conventional interactions

Standard ways of specifying the forces to be shown on Free Body Diagrams; they save you having to work them out for yourself (some of them are explained in videos on the Padlet wall in the Experiments step)

Couple

A special kind of twist that has no out-of-balance forces associated with it. It can be generated, for example, by two equal and opposite forces separated by an appropriate distance.

Creep

The continues extension of an object while under a steady load.

D

Damping

A mechanism by which energy is dissipated (lost) in a vibrating system (eg dry friction, fluid friction, hysteresis). Without damping a system would vibrate forever. You can have systems with very low damping, but an undamped system is not physically possible (2nd law of thermodynamics). But that doesn’t stop engineers from using them for enlightenment and analysis.

Density

Mass per unit volume, units are kg/m3.

Dry friction

The engineer’s approximate model for unlubricated contacts.

Dynamic deflection

The extension of an elastic member (eg a spring) when the load is moving. It will be an instantaneous value that is always changing (eg a spring loaded with a weight and caused to vibrate up and down.). (see also static deflection).

E

Encastré beam

A beam that is built in to a wall.

End fixing moment

The couple that restrains rotation in a built in end.

Exponential function

A mathematical function that is available on most scientific calculators.

F

Factor of safety

Factor of safety is defined in various ways, but is generally the estimated capacity of an item divided by the design load, ie how much capacity you have in hand to account for unknown or unexpected circumstances.

Force

A mysterious interaction between objects. It is a vector quantity. Accelerations are caused by forces, as are deflections in elastic members. The SI unit of force is the Newton, derived from mass, distance and time. It is the force that would give a mass of 1 kg an acceleration of 1 m/s2.

Free-body-diagram

An aid to visualising forces acting on a body when applying Newton’s laws. It is a diagram that identifies the body (or assembly ) to be analysed and shows it free of its surroundings. Where the surroundings interacted with the body, force arrows (and where appropriate arrows representing couples) are added. These arrows are often red.

G

Gravity

One of the four fundamental forces of nature. Gravitational waves were first detected in 2016, as predicted by Einstein. But we’ll stick with Newton’s understanding, which is fine for engineering tasks on Earth. We call the gravitational force on an object its weight and calculate it as the product of mass and the acceleration in free fall due to gravity, or W=mg. With mass in kg and acceleration in m/s2, the force is given in Newtons.

Ground clearance (of a car)

The distance between the ground and the lowest part of the body of a car. It’s one measure of the car’s ability to travel over uneven ground.

I

Impulse

The product of force and time - a vector quantity.

L

Length

The distance between two points, as measured by a rule for example. The base SI unit of length is the metre, currently defined in terms of the speed of light.

Linear characteristic

This appears throughout engineering. For an elastic member this means that the load-deflection relationship can be represented by a straight line.

M

Mass

A measure of the quantity of matter, independent of its volume. The base SI unit of mass is the kilogram, currently defined relative to a lump of platinum-iridium.

Matter

Stuff that the world is made of.

Moment

This is the most general word for the twisting effect of a force. The moment due to a force is given by the product of moment arm and force. In 2D it has a sense (clockwise or counter clockwise for example) as well as a magnitude. With force in N and distance in m, units are Nm.

Moment arm

In two dimensions (planar) this is the perpendicular distance from an axis to the line of action of a force. It is used when calculating the moment due to a force.

Momentum

The product of speed and velocity - a vector quantity.

N

Natural logarithm

A mathematical function that is available on most scientific calculators.

Newton’s Laws of Motion

Law 1 A body remains stationary (or moving in a straight line at constant speed) unless it is acted upon by a resultant force.
Law 2 Newton’s version is in unfamiliar terms, but modernised it becomes: an impulse applied to a body results in a change in momentum; it’s a vector relationship.
Law 3 To every action there is an equal and opposite reaction.

P

Particle

A theoretical object which has mass but is vanishingly small.

Pin joint

A connection between two bodies that permits relative rotation but not relative translation.

Power

The rate of doing work.

Projectile motion

Motion of an object under the action of gravity after being given an initial velocity.

R

Rectangular component

One of a pair of components at right angle to each other.

Resultant

A force that can replace the action of a set of components.

Rigid body

An object in which the constituent particles remain a fixed distance apart.

Roller support

A connection between two bodies that permits relative translation while supporting a compressive force.

Rolling friction

The resistance to motion of a rolling object (typically a wheel).

S

Scalar

A quantity that has magnitude only.

Set screw

A threaded component with a head in which the thread extends to the head. It is generally used in a threaded hole to clamp two items together.

Shear

A deformation of a body that is like the cutting effect of scissors. In mechanics of solids it describes a related deformation within a body.

SI units

A modern version of the metric system. with seven base units (such as time) and many derived ones (such as the Newton). They fit together so elegantly.

Simply supported beam

A beam that has a roller support at one end and a pin joint at the other.

Sliding vector

A vector that can be considered to act anywhere along its line of action.

Speed

In a straight line, it is the rate of change of position - a scalar quantity. In other situations it is the magnitude of velocity.

Statically indeterminate

A system of forces that you can’t solve by statics alone. Often you need a force-deflection relationship as well.

Static deflection

The extension of an elastic member (eg a spring) when the load is stationary (eg a spring loaded with a weight that is still) (see also dynamic deflection).

Stiffness at a point

Stiffness of a loaded elastic member (eg a spring) is the ratio (change in force)/(change in deflection). This is measured over the length of the member. If you have an assembly of springs (or are interested in how a body deflects under load) you can find the stiffness at any point by applying a load to that point and measuring the deflection there. Engineers are often interested in the deflection at points away from where the load is applied (for example the deflection of part of a car body when you apply a load at the engine mounts; often this is under dynamic conditions and can help in reducing vibration and noise transmission)

Stress

The stress on an object is the load divided by the area the load acts on. It can vary, in which case you would need to know the stress distribution, or perhaps the average stress.

Swage

A way of connecting a wire rope back on itself to make a loop, in which a deformable tube is compressed around both the main part and the free end .

T

Time

A dimension of existence, measured in Millennia, Centuries, Decades, Years, months, weeks, days, hours, minutes, seconds etc. The base SI unit of time is the second, defined relative to a particular characteristic of a caesium atom.

Torque

Another word for the twisting effect of a force. You generally find it in specific circumstances such as the output of a motor, or the twist generated by a spanner (“tighten to 50 Nm”).

Transducer

For the purposes of this course, a transducer is a device that converts a quantity you can’t measure into one that you can. Other definitions include: “a device that converts variations in a physical quantity, such as pressure or brightness, into an electrical signal, or vice versa (oxford dictionaries); and a device that converts one form of energy to another (Wikipedia).

Truss

A structure composed of two-force objects.

Two-force object

An object under the action of two forces only, in which the forces must be equal opposite and co-linear (in the same line of action).

U

The load at which a component breaks.

V

Vector

A quantity with both direction and magnitude.

W

Wall anchors

A device in which a bolt is fixed firmly in a brick or masonry of concrete hole, often by expanding a component into it; sometimes by chemical means.

Weight

Weight is the gravitational force on an object. We calculate it as the product of mass and the acceleration in free fall due to gravity, or W=mg. With mass in kg and acceleration in m/s2, the force is given in Newtons. See also gravity.

Work

Defined by the product of force and the distance moved in the direction of the force. It has mysterious aspects, such as being related to energy released when you burn fuel, and the energy involved in electrical systems.