Thames & Kosmos Structural Engineering: Bridges & Skyscrapers handleiding

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Handleiding

MOMENT

A force tends to cause an object to move. But depending

on where the force is applied on an object, the force can

also cause an object to rotate. For example, if you push

on the end of a wrench, the force causes it to turn around

the bolt. A moment is a measure of a force’s tendency to

cause an object to rotate around a speciﬁc reference

point. A moment is calculated by multiplying the force by

the distance between the point at which the force is

acting on the object and the reference point:

M = F × d

You can increase the moment by increasing the force or

the distance between the force and the point of rotation.

The more force... the more acceleration

(for the same mass)

F=ma

Reaction force

Action force

CHECK IT OUT

NEWTON’S SECOND LAW OF MOTION ...

... states that the sum of the forces on an object is

equal to the mass (m) of the object multiplied by the

acceleration (a) of the object, or

F = m × a

Both acceleration and force are vectors, so both can

change if either their magnitude or direction changes.

NEWTON’S THIRD LAW OF MOTION ...

... states that when a force is exerted on an object, the

object exerts a force equal in magnitude but opposite

in direction. In structural engineering, these forces are

called reaction forces. They typically occur at

connections or supports.

For example, all objects have a weight, which is

due to the force of gravity acting on them. If a book

is siing on a table, the book generates a force that

pushes down on the table due to its weight. The

table pushes back on the book with an equal and

opposite force. If there was no reaction force, the

book would fall through the table!

A moment is also a vector, but it is

represented by a curved arrow. The

direction of the arrow (clockwise or

counterclockwise) represents the

direction of the moment, and the

length represents the magnitude.

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