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2.5 The Effect Of Force

Definition of force

Force is  defined as anything that changes the state of rest or motion of an object moving in a straight line.

Force is a vector quantity  which has both direction and magnitude


The S.I. unit for force is N  or  kg m s-2


Force is measured by using a Spring Balance


The effects of a force


A force can

  • move a stationary object

  • stop a moving object

  • accelerate a moving object

  • decelerate a moving object

  • change the direction of  a moving object

  • alters the size of the object

  • alters the shape of the object




Newton’s First Law :            An object will remain at rest or continue with a constant speed in a straight line unless acted on by an unbalanced force.


Newton’s Second Law :     The acceleration of  a body   ,a,  is directly proportional to the net force acting on it, F , and inversely proportional to its mass, m.


Newton’s Third Law :         For every action there is an equal and opposite reaction.


The relationship  between  the acceleration,a ,the net  force ,F  and  the mass   , m.


Based on the Newton’s Second Law of motion,


                                a    a  F


                                a    a 1



we obtain              a    a  F



                         a  = k F



                          F = kma


In S.I. units , the definition of a force  of 1 N  is given as the amount of  force which is applied on a mass of 1 kg and which causes it to experience an acceleration of 1 ms2


                        1 N = k ( 1 kg ) ( 1 ms-2 )

Thus                    k = 1


Therefore         F = ma         

Where  F = net force

                                                              m = mass     

                                                               a = acceleration



Example 1



Two forces act horizontally to a block of wood  of mass 4 kg  as shown in figure above. What is the acceleration  and the direction of motion  of  the block.





Example 2


An object of mass 2 kg  is pulled on the floor by a force of  5 N and having a constant velocity.

(a)     What is the frictional force between the object and the floor.

(b)     Calculate the acceleration of the object if the object is pulled by a 17 N force?





Example 3


A bus of  mass 2 000 kg travels with a uniform velocity 

40 ms-1 for a distance 2500 m before it comes to rest. Calculate

(a)     the average deceleration of the bus

(b)     the average force applied by the brakes to bring the bus to  a standstill





Example 4


When traveling at  120 km h-1 a car has to overcome a drag force due to air resistance of  900 N. If the car has a mass of 1000 kg, determine the average force that the motor needs to apply if it is to accelerate at  5.0 ms-2.




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