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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 LAWS OF MOTION

 

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

                                          m

 

we obtain              a    a  F

                                           m

 

                         a  = k F

                                   m

                                    

                          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

Thus,

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

Thus                    k = 1

 

Therefore         F = ma         

Where  F = net force

                                                              m = mass     

                                                               a = acceleration

                        clip_image002

               clip_image004

Example 1

 

   clip_image005

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.

 

Solution

 

 

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?

 

Solution

 

 

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

 

Solution

 

 

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.

 

Solution

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