Download Topic 3 Foundation Engineering A Glossary

Glossary
Topic 3 Foundation Engineering “A”
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Unbalanced Force:
Newton’s Second Law states that force is directly proportional to acceleration and
mass. It is important to use the correct force in this equation. Draw a free-body
diagram and identify the net unbalanced force that is acting on the body – this is
the one that applies to Newton’s Law. Forces may be resolved in two directions so
the unbalanced force in each direction will then be needed.
Coherent System:
A coherent system of units is one where all the derived units are defined in terms
of fundamental units without any proportionality constants.
Reaction:
If body A exerts a force on body B then, by Newton’s Third Law, body B exerts
and equal but opposite force on A – this is called a reaction. A mass suspended
from a spring balance exerts a force on the spring. When the mass is stationary
then the spring exerts an equal and opposite reaction on the mass.
Free-Body Diagram:
When applying Newton’s Second Law it is recommended that a free-body diagram
is drawn. This drawing must show the body or object of interest and all the
external forces acting on it. As far as acceleration is concerned of interest is the
net unbalanced force which is directly proportional to the acceleration through
Newton’s Second Law.
Hooke’s Law:
Hooke’s law applies to a spring either in tension or compression. Hooke’s Law
states that the force exerted by the spring (spring force) is directly proportional to
the extension (or compression) of the spring from its natural length. The constant
of proportionality is the spring constant.
Apparent Weight:
The apparent weight is the actual weight an object experiences when acted on by
positive or negative forces in addition to gravity when resolved in the y-direction.
The apparent weight may be greater or smaller than the true weight of the object,
see “True Weight” in this glossary.
True Weight:
The true weight of an object is the weight the object experiences when no other
forces, except gravity, are acting in the y-direction, see “Apparent Weight” in this
glossary.
Co-linear:
Co-linear is a term used when all the forces are aligned to the same line of action.
The forces may all point in the same direction or in opposite directions. A freebody diagram helps to account for the sign of the forces so that the magnitude
and the direction of any net force may be found.
Tension:
The tension in a cable is the force exerted by the cable on an object. By Newton’s
Third Law the object also exerts an equal and opposite pull (or reaction) on the
cable; thus tensions occur in pairs and act to pull the cable apart. If the free body
diagram applies to the object, only the external force acting on the body should be
shown. Thus, only the original pulling tension on the body is shown – the reaction
will not be shown, unless a free-body diagram for the cable itself is of interest.
Force of Friction:
The force of friction always operates in opposition to the direction of motion.
Friction may be increased or decreased but never completely eliminated. Friction
is proportional to the normal force exerted by the surface on the object and the
constant of proportionality is the coefficient of friction.
Coefficient of Static Friction:
The coefficient of static friction is the ratio of the frictional force between two
surfaces divided by the normal force exerted by the surface on the object when
the object is stationary.
Coefficient of Kinetic Friction:
The coefficient of kinetic friction is the ratio of the frictional force between two
surfaces divided by the normal force exerted by the surface on the object when
the object is moving. The coefficient of static friction is always greater than the
coefficient of kinetic friction – it takes a greater force to start an object moving than
it does to keep an object moving.
Momentum:
Momentum of a body is the product of the velocity of the body times the mass of
the body.
Momentum Conservation:
The momentum of a body, when not acted on by an unbalanced force, is
conserved. If two or more moving objects collide the total momentum of all bodies
before and after the collision must be the same.
Impulse:
The impulse of a force is the product of the force acting on a body times the
duration of that force. Often short-acting forces are variable and an average force
may be used.3