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Forces - introduction.

posted Dec 5, 2015, 11:31 AM by Upali Salpadoru   [ updated Jun 21, 2021, 10:41 PM ]


The car is resisting, yet it may move.

A force has a direction. It is a vector.

Fig.1.  Frank is pushing a car.


Who will win?

One who gives the greater force will win. What may happen if they are equal?

Force has a value Magnitude.

What exactly is a force?

It is usually defined as ;

The effect that can cause a change in the motion of a mass if not balanced.

"Hey! " Then what is mass?"

Fig.2. Kate and Nelly are pulling each other.

 Mass is the quantity of matter in a body.

Exception:- In the case of Kate and Nelly if the two forces are balanced there may not be a visible movement, but the rope will experience a Tension Force.

Push and Pull are two forces exerted by muscles.

What are other forces?

Reaction forces.

     "Every action has a reaction".

The pup exerts a force on the table.
Table exerts a force up.
 Man pulls the tree.
The Tree pulls the man. 
   Copter pulls the man up.
 Man pulls the copter down.
Fig.3 Reaction forces.
 This is a force that originates due to a  force.  Every force has a partner force. Forces appear in pairs.

The force by the pup is the pulling force of the earth. We call this weight. So weight is a force.

But there is no visible movement.  It is because the weight is balanced by a force that the table gives. Table gives a reaction ; we call this a support force.

  Support force is a reaction force. All reaction forces are not support forces.

Fig.4 Lily is pulling a rubber band.

Rubber band is pulling the girl. This is a reaction but not a support. This is due to the elastic nature of rubber. So it is Elastic force.(related to tension)

Every action has an equal and opposite reaction.


This is Newtons third Law.

The pushing force that results due to an engine is termed Thrust.

Several forces are marked by coloured arrows in this figure.

Fig.5 Forces on a car

Practice problem.

    Highlight to get answers.



Name of force.

  1. Down



     2.   Right



     3.   Up.


Support (Reaction)

     4.   Left


Air resistance.


A force due to the flow of air currents, 

This principle states that an increase in the speed of a fluid causes a reduction in pressure. 


As the plane gives a forward thrust Air flows very fast over the wings. This produces a low pressure area above which pulls the plane up.

Refer. Bernoulli principal.

Fig. 6. Lift due to air currents.

Fig. 6.FFig.8. Fig.7.Forces on a plane. Fig. 8. A gliding bird.



Fourth mentioned Air resistance is one type of a force coming under the broad category of forces known as Friction.

Road friction actually helps to give the thrust.


Not an easy task. Eh!

Fig.9. Types of Friction.

What is Friction?

Friction is a force that resist movement. It acts in the opposite direction to movement.

Now you know seven forces ? Is that all? No!

  1. Push  2.Pull, 3 Thrust 4. Reaction, 5. Friction 6. Tension  7. Lift.

There are a few more. All the forces we have named so far, except weight, are contact forces. That means there is some connection between the object giving the force and the one receiving the force.

An object on an inclined plane / Ram.


What do you know about 3 and 4? They are equal.

2 and 5? 2 is greater than 5,

highlight to get answers.

Fig.10. Forces on an inclined plane.

Non Contact forces.- Action at a distance

                             non contact.jpg

Fig.11. Non contact forces.

  1 Force of Gravity.

Weight is a non-contact force.

Earth pulls all matter towards it. This force extends even beyond the moon.

If not for this force the moon will go away far into the unknown space. All matter show this property. For normal objects it is so small and we do not realise it.

Fig. 12. Earth attracts the Moon and the moon attracts the earth.    

As there is no material connection between the two bodies we call this a “non contact- force”.

This is Force of gravity.

Weight is what results from the force of gravity.


Any mass can attract any other mass, but this is a very weak force.

It increases with mass and reduces with distance. It is given as below.

"This is directly proportional to the product of their masses while inversely proportional to the square of the distance." F ∝ M x m / r 2

2. Magnetic.


Fig. 13.In magnets unlike poles attract while like poles repel.

In this experiment the magnets are kept on torch batteries to reduce friction.

3.  Electrical Attractions and Repulsions.

Get a plastic comb. Brush your hair several times. Then hold it above pieces of light paper.comb.jpg

Note what happens?

Wait for some time and see what happens.

Why O Why?

When you rub dry hair with a comb some electrons move into the comb.  This will give a negative charge.

A charged body can exert an attractive force on neutral bodies.

E charges.jpg

If the pieces of paper gets some of the extra electrons from the comb they will repel.

Fig. 14. Attractions and repulsions due to electrical charges.

3.  Gravitational Repulsion.    There is no gravitational repulsion

Yet in the Universe the Galaxies are accelerating away from each other.

This has not been explained properly.

Forces and Sir Isaac Newton.

Although Galileo made some demonstrations with regard to forces, it was Newton who produced a mathematical explanation to understand what a force is.  

He showed that a Force can be measured by multiplying mass by the acceleration it will produce.

 Force = Mass X Acceleration.  newton = kilogram x acceleration due to gravity     N = m x g
This is why the unit for measuring a force has been named as a newton.

        Some interesting Activities.

  1. Get familiar with these forces too.
     Spring force, Elastic force ,  Capillary force. Upthrust.

   2.  In society, forces are known by various names. There are a multitude of such names; for example a kick is a force executed by a leg. Drop is making use of the force of gravity.

In fact we can name a force starting with almost every letter in the alphabet.
A few examples are here.  Try to complete this.

 A=    Angling is a favorite of  the Elderly.. 
B=    Sangakkara batted the whole session.
C=   Catch the ball with both hands. (You have to give a force to stop the ball)
Drill a hole before you insert a screw.
E=    "Erect a statue in his memory.". 
F=    "Flip it open".
G=    ..........
I=     ..................
J=   Jump up or down.
K=    ................
M=   Playing chess we move the pieces.
 Y=    ................ 
You have to pierce the ear to wear ear studs.
Q=   ........................
R=    Roam the jungle and find the plant.
S=    .....................
T=    torque.
U=    ...........
Z=     Zip the bag or else the keys may fall.

Free body Force Diagrams.

A force can be represented by straight lines drawn to scale.

      The girls pulling force is balanced by the elastic forces of the rubber bands. As the net force is zero there is no movement, but the rubbers will be stretched due to tension.
      The moment she takes the hands off the missile will get forced out by the rubber bands.
      The force will be gives only up to the extension of the rubber bands, but the missile will go a long way without a pushing force.
      This shows that an object can move without a net force.

According to the free body force diagram, what should happen to the toy plane?
It must go up as well as move to right. Highlight to get the answer.

     Multiple Choice  questions.

   From Q  1 to  4.
                                 Answers:   A.- Weight,  B.- Push   C.- Friction D.- Support.

1.             2.           3.             4.      


    From Q  5 to  13.
                 Answers.   A.- up thrust,  B,- Air resistance  C.- Water resistance.  
                                 D.- Thrust.  E- weight.    F- Lift.

   5-1.        6-2.          7-3.

   8- 4         9.-5.


  10.-1.       11.-2     12.- 3



14.  Convert 100 km/h to meters per second.
               A.-  (100 x 1000) / 60 x 60.       B.-  100 x 100 x 60 x 60.
               C.-  (100 /1000) / 60 x 60         D.- (1000 /100) / 60 x 60.
    15.    Which statement is not correct?
           A.- A force can accelerate a body.    B.-  A force can decelerate a body.
           C.- A force is not required for motion.   
           D.- No force can effect without contact. 
                                                                                          2 x 15 = 30 marks 

         Answers.- 1. C,  2. B,  3. D.  4. 1.                              5.  D,  6. B,  7. C,  8. A,
                                       9.   E,  10. E, 11, D,  12. E, 
                                      13.  B,  14.  A.  15. D.
                                                                              Highlight to get the answers.
        Other Questions.
   Q 1.0

Name the 5 forces marked in the diagram.
1.- Weight.  2.- Sliding component due to weight.   
3.- Normal component due to weight.  4.- Normal.  
5. Friction.

                                                                                                              2 x5 = 10 marks.
    Answer the questions with regard to the cyclist in the picture.


      2.1   What force is acting down?
      2.2   What force is balancing it ?
      2.3   What is the advantage of using rubber                      tyres?
      2.4   Why does he need a greater effort to                        climb?
      2.5   Why does he need a greater effort if he were to maintain a higher constant                  speed?
                                                                                            2 x5 = 10 marks.
              1. Weight.    2. Support force.  3. To increase friction.
              4. To overcome the weight.   5. At high speed air resistance is high.
        Following is a list of 20 words.  
        Select them under three categories as Matter,  Energy, Force.
          Glass,   wood,  ,  Flame, Oxygen, Drag,  Light.  Electricity, sound, Heat, 
          Radio wave, Plant,         - Throw,  Lift,  Tension, Weight, Thrust, Press,                        repel.  Thread,   Rubber, 
                                                                                                   20 marks.
    Matter:-  Glass,  wood,  Flame,  Oxygen. Plant,  Rubber. Thread,
    Energy:-  Light, Heat, Radio waves, Sound, Throw, 
    Forces:-   Drag,  Throw.  Lift,  Tension, weight, Thrust, Press, repel, 

    Q 4.0    
           A train, 100 m. long has a velocity of 80 km/h. 
            4.1 covert this speed to meters per second.
            4.2  What time will it take to pass a signal post?
            4.3  What time will it take to pass a flat form 80 m long?
            4.4  If the train stops in 30 seconds what is the negative acceleration.
            4.5  If the train has to pass another train, of the same length and speed,  in the opposite direction,  what is the time it will take 
                                                                                                              5 x 6 = 30 marks.
                 Answers:-  4.1   
                  The distance 80 km =  80 x 1000 m.  
                  Time 1 hour  .........=  60 x 60 seconds.           
                   In 60 x 60 s the distance =   80 x 1000 m    
                   In 1 second  the distance =  80 x 1000
                                                                60 x 60
                                             That is   =  800 divide 36 m
                                             Speed    = 22.22 ms-1.
                                   4.2      22.2 m take only  = 1 second.
                                            Time for 1 meter    =  1 /  22.2  s.
                                          Time for  100 meter . =  100 / 22,2 s.
                                                                          =4.5 seconds.
                             4.3        Total distance  =  100 + 80 m.
                                          The to  travel 180 m =  180 m. / 22.2 s.
                                                                          =  8.1 s s
                              4.4       Acceleration      = Change in velocity ÷  Time.
                                                                 =  80 ms-1 ÷  30 s.
                                                                 = 2.67 ms-2.
                             4.5      Relative speed of two trains =  22.2 ms-1 + 22.2 ms-1.
                                                       44.4 meters take  = 1 second.
                                                               200 m take   =  (1/44.4) x 200 s.
                                                                                   = 4.5 seconds.

  v= d/t