As defined before, force is the action of one body on another.

It is a vector quantity since its effect depends on the direction as

well as on the magnitude.

The effect of the force applied to the bracket depends on , the

angle q and the location of the point of application of .

P

P

P

q

We can separate the action of a force on a body into two effects

as external and internal. External effects are the reactive forces

exerted on the bracket by the wall. Forces external to a body can

be either applied or reactive forces. Internal effect is the

resulting internal forces and deformations distributed

throughout the material of the bracket.

P

mgW yF

xF

M

Forces are classified as either contact or body forces.

A contact force is produced by direct physical contact; an

example is the force exerted on a body by a supporting surface.

A body force is generated when a body is located within a force

field such as a gravitational, electric or magnetic field.

An example of a body force is your weight.

Forces may be further classified as concentrated or

distributed.

Every contact force is applied over a finite area and is therefore

a distributed force.

However, when the dimensions of the area are very small

compared with the other dimensions of the body, the force may

be considered concentrated at a point.

The weight of a body is the force of gravitational attraction

distributed over its volume and may be taken as a concentrated

force acting through the center of gravity.

concrete

Concentrated force

Distributed force

Concrete (Beton)

According to Newton’s third law, the action of a force is always

accompanied by an equal and opposite reaction. It is essential

to distinguish between the action and the reaction in a pair of

forces.

To do so, we first isolate the body in question and then identify

the force exerted on that body (not the force exerted by the

body).

Concurrent forces are forces whose line of action all pass through a common point.

The force vectors are all in the same plane.

The line of action of the forces are parallel to each other.

All the forces have a common line of action.

FB

FA=FBFA

A tensile force applied on a body will tend to stretch or elongate the body,whereas a compressive force will tend to shrink the body in the direction ofthe applied force.

F

F

F

F

Tensile Compressive

Let’s consider two bodies which are in contact. The force acting on body 1 from

body 2 is . can be divided into two components as a normal force , drawn

perpendicular to the tangent line at the point of contact and force , drawn

parallel to the tangent line.

F

F

fFN

(1)

(2)

tangent

(2)F

N

fF

(1)

tangent

F

N

fF

is named as the normal component of the contact force and is named

as the friction component of the contact force. If the contacting surfaces

are smooth, then can be neglected ( ); but if the contacting surfaces

are rough it has to be taken into consideration.

N

fF

0fF

fF

The relationship between and is given by

Ff = mN

where m is a dimensionless coefficient of friction varying between 0 and 1.

N

fF

(1)

tangent

F

N

fF

tangent

(2)F

N

fF

T

Forces in strings, cables, etc. are always taken along the string, cable, etc. and

their direction always points away from the body in consideration. They

exert force only when they are tight. When loose they exert no force. Hence,

they always work in tension. Usually their weights are neglected compared to

the forces they carry or support.

BCT

ADT

Pulleys are wheels with grooves that are used to change the directions of belts

or ropes and generate a higher output load with a much smaller input force.

Unless stated otherwise, or apparent from the problem, the tension forces at

both sides of a belt are taken as equal. They are equal as long as the belt does

not slide on the pulley, and the pulley rotates freely with a constant

velocity.

Spring force is always directed along the spring and is in the direction as if to

return the spring into its undeformed length.

Fspring=kx (Spring force)

(k: spring constant, x: deformation of the spring)

Fspring

P

F=kx

F=kx

F=kx F=kx

F F

x1 x x2x1 x x2

F=kx F=kx

Unstretched Stretched Uncompressed CompressedPosition Position Position Position

x x

Stretched Compressed