EQUILIBRIUM OF

CONCURRENT

FORCES

To analyze the forces at a point

using the first condition of

equilibrium.

To apply the concept of friction to

the first condition of equilibrium

Objectives

a vector quantity

an interaction between two

objects or between an object and

its environment

changes the dimension or shape

and/or motion of the object

(translational or rotational)

Force (F)

Metric System

MKS kg m/s2 or N

CGS g cm/s2 or dynes

* 1N = 1X105 dynes

English System

FPS slug ft/s2 or pound

Units of Force

Contact Forces

acts on the interacting objects

which are physically connected

with each other

ex. Friction, Tension, Compression,

Normal Force, Air Resistance ,

Applied Force, Spring Force  

Two Categories of Forces

Applied Force (Fapp)is the force

applied to an object by another

object

Normal Force(Nf) is the support

force exerted upon an object in

contact with another stable

object

Friction (Fk or Fs) is the force

exerted by a surface as an

object moves across it

Air Resistance(Fair) is a special

type of friction that acts upon

object as it travel through the

air

Tension(T) is the force that is

transmitted through a string,

rope, cable, or wire when it is

pulled tight from both ends

Compression (C) is an inward

force on an object

Spring Force (Fspring) is the

force exerted by a

compressed or stretched

spring upon any object that is

attached to it

Non- contact Forces

acts on the physically

separated objects

ex. gravitational force , electric

force, magnetic force

Gravitational force, Force of

gravity, Gravity Force, Gravity,

Weight is the force with which

the earth, moon, or other

massively large object attracts

another object towards itself

Fgrav or W

Magnetic Force is the

attractive or repulsive force

exerted on a magnetic pole

Electrostatic force is the force

between 2 charges at rest

Mass is the amount of matter

of the object

Weight is the pull of the earth

(or any other planet) upon the

object; dependent upon the

value of g

W = mg

where m is the mass of object

g is acceleration due to

gravity

g = 9.8 m/s2

= 980 cm/s2

= 32 ft/s2

*If forces(contact or non-

contact) act on a common

point of an object, these are

concurrent forces

*If forces act on different points

of an object, these are non-

concurrent forces

Newton’s Three Laws of Motion

Newton’s First Law of Motion

(Law of Inertia)

“An object at rest remains at rest and

an object in motion continues to

move with constant velocity in a

straight line.”

Newton’s Second Law of Motion

(Law of Acceleration)

“If a net force (resultant) acts on an object, it will be accelerated. The magnitude of the acceleration will be proportional to the net force and inversely proportional to the mass of the object and the direction of the acceleration will be in the same direction as the net force.”

Newton’s Third Law of

Motion (Action-Reaction)

“To every action, there is

always an opposed and equal

reaction.”

Equilibrium

*no net change; balance

*no translation or rotation

*static equilibrium

*dynamic equilibrium

First Condition of

Equilibrium

The resultant of forces

acting on an object is zero.

Two Conditions of Equilibrium

0

0

0x

y

F

FF

Second Condition of

Equilibrium

The sum of all the torques

acting on an object is zero.

Free-body diagrams

are diagrams used to

show the relative magnitude

and direction of all forces

acting upon an object in a

given situation

Find the tension in the cable and the

magnitude and direction of

compression exerted on the strut

by the pivot in each of the ff.

arrangements. Let the weight of

the suspended object in each case

be 1000N. Neglect the weight of

the strut.

1.

30o

2.

30o

3.

60o

45o

4.

30o 45o

Find the Tension in the ropes A and B.

45o

A

B

500 N

If the mass of the block is 5 kg, what is its weight?

T = ?

If the weight of an object is 95 pounds, what is the mass in slug?