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Force: Force is an action of one body on another body.

In Dynamics a force is defined as an action which tends to cause acceleration of a body.

Force is a vector quantity.

An Example: 

Effect of action depends on: 1) magnitude 2) the angle 3) the location of point of application

Effects of Force: Action of force on a body has two effects:

1) External, like reactive forces in response to applied forces. 2) Internal, internal forces and deformations distributed throughout the material.

Force Classifications:  

1) Contact Force: force produced by direct physical contact. 2) Body Force: force generated by virtue of the position of the body with in force

field, like gravitational, electric or magnetic force, example: weight of a body.

1) Concentrated Force: 2) Distributed Force: example, weight

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Principle of Transmissibility: 

 

Concurrent Forces: Two or more forces are said to be concurrent at a point if their lines of action intersect at that point.

Rectangular Components: 

 

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Example: 

 

Moment: In addition to the tendency to move a body in the direction of its application, a force can also tend to rotate a body about an axis.

The axis may be any line which neither intersects nor is parallel to the line of action of the force.

This rotational tendency is known as the moment M of the force. Moment is also referred to as torque.

c o ss i n

x y

x x

y y

x

y

F F F

F F

F F j

F FF F

i

θθ

= +

=

=

==

1

1

600 cos35 491

600 sin 35 344x

y

F N

F N

= × =

= × =

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Moment about a Point: The magnitude of the moment or tendency of the force to rotate the body about the axis O-O perpendicular to the plane of the body is proportional both to the magnitude of the force and to the moment arm d, which is the perpendicular distance from the axis to the line of action of the force.

Therefore, magnitude of the moment is defined as, M = Fd

The Cross Product: The moment M of a force F about a point A may be represented as cross product: M = r x F

Where r is a position vector which runs from the moment reference point A to any point on the line of action of F.

Magnitude is given by, M = Fr sin α

Right Hand Rule for Sense of Moment: 

 

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Varignon’s Theorem: It states that the moment of a force about any point is equal to the sum of the moments of the components of the force about the same point.

Couple: The moment produced by two equal, opposite, and non collinear forces is called a couple.