Glossary
Topic 5 Foundation Engineering “A”
Peter,
Yes it’s glossary time again……………..
Angular Acceleration:
Already done
Tangential Acceleration:
See also “Angular Acceleration” in this glossary. The tangential acceleration
(m s-1) is linked directly to the angular acceleration (rad s-1); in fact the tangential
acceleration is simply the angular acceleration times the distance to the centre
(radius). On a rotating circular disc, which is either speeding up or slowing down,
all the points along any radius will have the same angular acceleration; however,
each point on the radius will have a different tangential acceleration, since each
point will be a different distance from the centre.
Centripetal Acceleration:
See also “Angular Acceleration” and “Tangential Acceleration” in this glossary.
The centripetal acceleration is nothing to do with changes in angular velocity that
result in angular and tangential acceleration. When a body is rotating at constant
angular velocity its direction is changing, it therefore follows that it must be
accelerating. This acceleration acts towards the centre of the circle and is called
centripetal acceleration.
Centripetal Force:
If a body is rotating in a circle at constant angular velocity its direction is
continuously changing so it must be accelerating. This acceleration is directed to
the centre of the circle and is called centripetal acceleration; see “Centripetal
Acceleration” in this glossary. However, by Newton’s Second Law this
acceleration must be caused by an unbalanced force. This force must also point
to the centre of the circle and is called the centripetal force.
Moment:
The moment of a force is the product of the force times the perpendicular distance
and in SI it has units of (N m). Care should be taken not to substitute joules,
because a moment of a force is a turning effect and not energy. What makes this
different from energy is that the distance is the perpendicular distance from the
line of action of the force to the axis of rotation, not the distance through which the
object is moved. Moments can be clockwise or anticlockwise.
Torque:
Torque is another name given to the moment of a force; see “Moment” in this
glossary.
Lever:
A lever is a practical device that makes use of the principle that if there are
upward and downward forces acting on a beam, which can pivot around a fulcrum
then, if the beam is in equilibrium, all the moments must sum to zero. A level
enables an operator, using a small effort, to dislodge a much larger load.
Couple:
A couple is where a pair of equal and opposite forces act at either end of a device
that is free to rotate. The forces are parallel to each other and produce a turning
effect or torque that is proportional to one of the forces (they are equal) and the
perpendicular distance between them. The torque has units of (N m); see
“Moment” of a force in this glossary.
Moment of Inertia:
Moment of Inertia for a body in rotary motion is analogous to mass for linear
motion. Just as mass is the resistance to linear acceleration, so moment of inertia
is the resistance to angular acceleration. The term “inertia” is used because it
means resistance to change. A tightrope walker uses the moment of inertia of a
long pole to resist rotation and assist balance.
Angular Momentum:
Just as the term momentum applies to linear motion, so angular momentum
applies to angular motion. Angular momentum is equal to the product of the
moment of inertia and the angular velocity when a body is rotating about some
axis. The units of angular momentum are (kg m2 s-1) which can also be expressed
as (N m s). See also “Conservation of Angular Momentum” in this glossary.
Moment of Inertia:
Just as linear momentum of a body is conserved, so long as the body is not acted
on by an unbalanced force, so also angular momentum of a rotating object is
conserved, so long as the rotating body is not acted on by an unbalance torque.
Angular Work:
If a force moves through a distance then work is done (N m); joules (J) may be
used since work is a form of energy. If a torque moves through some angular
displacement then angular work is done, also measured in (N m).
Rotational Kinetic Energy:
Kinetic energy associated with linear motion, also called translational kinetic
energy, is the energy stored in a moving body. Kinetic energy is directly
proportional to both the mass and the velocity squared; it has units of joules (J).
Kinetic energy associated with angular motion, called angular kinetic energy, is
energy stored in a rotating body. It is directly proportional to both the moment of
inertia of the body and the angular velocity squared; it also has units of joules (J).