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2.9 Equilibrium

Forces in Equilibrium

The principle of the forces in equilibrium states,

“ When forces act upon an object , the object is said to be in a state of equilibrium when the resulting force acting on the object is zero ( no net force acting upon it) ”

When the equilibrium is reached, then the object is in two states, that is

(i)      remains stationary  (if the object is stationary)

(ii)     moves at a constant velocity ( if the object is moving)

Based on   , F = ma  atau  a = F


When the equilibrium of forces is achieved, then, F = 0  ,  hence  a =0

Thus a = 0  , it means the object remains stationary or moves at a constant velocity.


Newton’s Third Law of Motion

Newton’s third law of motion states , “ To every action there is an equal but opposite direction”


Examples Forces in Equilibrium



                                Weight = Normal reaction



                                Weight = Tension



                                            Buoyant force = Weight





                                     Weight = Normal reaction




Weight = Normal reaction

Pulling force = Frictional force




                                Weight = Lifting force

                                Driving force = Dragging force



Weight = Normal reaction

Engine thrust = Air resistance + Frictional force




                Buoyant force = Weight of load + Weight of helium gas


Two Forces in Equilibrium



                                                                          P  +  Q = 0

                              We can rewritten into  P  = – Q


Example 1

Figure shows a stationary wooden block of mass 2 kg resting on a table.



(a)           the weight of the wooden block

(b)           the normal reaction



Three Forces in Equilibrium



P  +  Q  + R = 0


When three forces in equilibrium the  triangle of forces in one direction (in order)

Example 2


The following figure shows a steel sphere of mass 12 kg suspended from a length of rope which is pulled to the side by a horizontal force of  M. The tension of another rope is N.


(a)           Draw a  triangle of forces.

(b)           Calculate the value of

                (i)            M

                (ii)           N






Resultant force


Force is a  vector quantity  and hence it has magnitude and direction.

Two or more forces  which act on an object can be combined into a single force called the resultant force.


If  two forces are in same line, vector addition is easy. We simply add the forces if both pull or push together;

subtract  them if one is in the opposite direction.


If they are at an angle, the resultant force can be determined by the triangle method and the parallelogram method.


Parallelogram method :




In this method the tail of the first vector is joined  to the tail of second vector and then draw a parallelogram.

The diagonal represents the resultant force.


Triangle method:




In  this method the tip of the first vector is joined  to the tail of second vector and then draw a  line to complete the triangle.

The third side represents the resultant force.


Example 3


Find the resultant force for the following figure:-










Example  4


The figure shows a trolley is pulled by two forces


What is the magnitude and the direction of the resultant force acting on the trolley.





Example  5


Figure shows a boat is pulled by  two forces.

Calculate the magnitude of the resultant force acting on the trolley.











Resolution of  forces


A force can be resolved into two components, that is,

 (i)           the horizontal component, Fx and

(ii)           the vertical component , Fy


                                Fx  = F cos q

                                Fy  = F sin q


·         q  is an angle between the force F to the horizontal line

·         the sign of the force depend on the quadrant where the force , F is placed


For an object on a inclined plane, the weight,W of the object can be resolved into two components ;

(i)            parallel to inclined plane, A

(ii)           perpendicular to inclined plane,B





A = W sin q

 B = W kos q


Example  6


Find the values of Px and Py for the following figures.












Example 7


Figure shows a stationary wooden block of mass 50 g which is placed on a inclined plane that is at an angle of 40o to the horizontal. 

What is the magnitude of the weight parallel to the inclined plane.




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