[Professional Knowledge Base] Concepts Related to the Power of Engineering Mechanics (1)

Rigid body

Under the action of force, the distance between any two points inside it remains unchanged, that is, a rigid body is an object that does not deform. A rigid body is a simplified and abstracted ideal model.

Force

Force is the mechanical interaction between objects, the effect of which is to change the mechanical motion turntable of the object, or to deform the object.

The effect of a force on an object depends on three factors of the force: magnitude, direction, and point of action (line of action), usually referred to as the three elements of force.

A force is a directional quantity (a vector).

Unit of force

In the International System of Units (SI), the unit of force is the New [ton] and the symbol is Newton (N) or Kilonewton (kN). In the engineering unit system, the common unit of force is the kilogram-force or ton-force, symbolized as kilogram-force (kgf) or ton-force (ft).

The conversion relationship between the two is: 1kgf=9.8N

Free body

An object is called a free body if it can move arbitrarily in space. Such as flying planes and artillery shells.

Non free body

Objects that cannot move in certain directions due to the constraints of other objects around them are called non-free bodies.

Constraint

Surrounding objects that limit the motion of a non-free body are called constraints.

Restraint reaction

The effect of restraint on non-free bodies is actually a force, which is called restraint reaction force, or reaction force for short.

Two force rod

A component that is only in balance under the action of force at two points is called a two-force component or a two-force rod. The two forces it receives must be along the line connecting the points of action of the two forces, and are equal and opposite.

Moment

The mechanical effect of a force causing an object to rotate around a point is called the moment of the force to that point, or torque for short.

Turn the nut like a wrench.

The rotation point o is called the centroid, and the vertical distance d from the centroid (point 0) to the line of action of the force F is called the force arm of the force F to the point o.

Since the rotation has two directions, clockwise and counterclockwise, the positive and negative signs are generally used to indicate the direction of rotation.

The moment of force-to-point is an algebraic quantity, its absolute value is equal to the product of the magnitude of the force and the force arm, and its sign usually stipulates that when the force makes the object turn counterclockwise around the centroid, it is positive, and vice versa.

The moment of force to a point is represented by the symbol M0(F).

When the line of action of a force passes through the centroid, its moment to the centroid is zero; when the force moves along its line of action, the moment of the force to a point remains unchanged. The unit of moment is usually Newton*meter (N*m) or kilonewton*meter (kN*m).

 Couple & Couple Moment

A force system composed of two parallel forces that are equal in magnitude, opposite in direction and not collinear is called a force couple, denoted as (F, F').

The effect of a force couple on an object is to make the object rotate. The rotational effect of a couple on an object can be measured by the moment of the couple, and by the algebraic sum of the moments between the two forces of the couple at a point in the action surface.

The vertical distance d between the two forces of the couple is called the couple arm, and the plane where the couple is located is called the action surface of the couple.

The moment of the couple to point o is denoted as Mo(F, F'), and the centroid o is optional. It can be seen that the action effect of the couple depends on the magnitude of the force, the length of the couple arm and the direction of the couple, and has nothing to do with the centroid.

Therefore, the product of the magnitude of the force in the couple and the couple's arm and a positive and negative sign is called the couple moment, which is denoted as M(F, F') or M for short.

Couple moment is an algebraic quantity whose absolute value is equal to the product of the force and the couple arm. The sign indicates the direction of the couple, and it is usually stipulated that the counterclockwise direction is positive, otherwise it is negative.

The unit of the couple moment is the same as the moment, which is also N*m or kN*m.

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