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Forces- Friction.

posted Nov 9, 2015, 1:11 AM by Upali Salpadoru   [ updated Jun 5, 2018, 3:23 PM ]

Fig.1  Why Are the satellites not streamlined like planes and cars?

We often use the term 'FRICTION'. Do we realize that it is a force, a vector quantity having a 

magnitude and direction? 

 It is a  force that comes into effect only when there is movement  or an inclination to move, rubbing 

or colliding matter against matter.

A rocket shooting in outer space will not experience friction as there are no material obstacles such as molecules of air.

 F measurer..jpg


Fig.2  Ali pulling a block of wood.

Ali  kept a block of wood on the table and pulled it with a spring balance. 

The spring balance readings and the observations are tabulated here.





Type of friction.


No movement



No movement.



No movement



still no movement.



The body moved !


What force prevents any movement  with a  force less than 5 N ?

This is nothing but  FRICTION.

Friction is the force that prevents or restricts  motion.

This is the force that we call sliding friction or contact friction.

 If you increase the pull the friction also will increase, but this cannot go on forever. In this experiment, 5 N is the maximum friction produced is between 4 and 5 newtons..

 The maximum friction between  two surfaces can produce at a time is called 

 “The limiting Static Friction”

Home Experiment   1.

Aim:...  To compare friction between different surfaces

Method :-   

What you need:
Yoghurt cup, an empty can, some thread , a few identical coins
A block of wood or any other material. 

Fig.4.  The diagram for the experiment.

Set up the apparatus as shown.

i. Place the block on a rubber mat or a carpet as shown. 

ii. Add weights to the cup. 

The can is held at the end to reduce friction. 

 If the edge of the table is round and smooth you do not need that.

iii.  Change the rubber mat to other materials and repeat.

Record the readings on a chart as given above.


The results may be best illustrated using a graph. Use the reaction on the X axis and the Maximum Friction on the Y axis.

Findings:- These will help you to generalize and draw conclusions from the results.



Are the substances having high friction hard or soft?


Are the surfaces having high friction smooth or rough?


Does the surface area have any effect on friction?


Tightness and Friction

Let us start this section with a question.

If Ali is pulling the ruler from Nelly's hand, and if she does not wish to part with it, what can she do?

"She can tighten the grip!", Absolutely right. She actually increased the force acting between the two surfaces.  You may investigate further by this experiment.



Home Experiment 2

Aim:....To investigate the change of friction according to tightening force.

Method :-

Fig.5 To investigate Tightness and friction.

For this experiment Ali used several identical blocks of wood. The lowest block is experiencing friction. Others are placed one by one to increase the weight. It could have even been identical books.

Recording of Results


The results may be best illustrated using a graph. Use the reaction on the X axis and the Maximum Friction on the Y axis.

Guide to Findings

 When the weight is doubled does the friction also double?

The Frictional force, divided by the Reaction Force between the two surfaces, which gives the tightness, is a constant value called the 'Coefficient of Friction, This is denoted by the Greek letter 'mew' . Investigate the validity of this Formula for two surfaces of your choice…

 Coefficient of friction = Frictional force/ Reaction .
 F / R = μ


Cause of Sliding Friction

 At least one of the reasons can be craggy nature of the surfaces when considering them at -microscopic level

 If a normal surface of a table, book or any other object is looked through a microscope it would be possible to see ups and downs. However mush we made them smooth by polishing they will not completely disappear.  

A practical method to close these pits would be to fill them up, This is what we call lubrication.

With what shall we fill them up?

Fig.6 The rough nature of surfaces is shown magnified.


Home Experiment  3

Aim:-  ...  Testing Lubricants.



Fig.7 Testing lubricants.

Use the same apparatus as before, slightly modified as follows: 

The block is kept on a tray on to which a solid or a liquid lubricant may be added.

Some examples:-  Cooking oil, Engine oil,  Grease, sand,  powder, Soap solution.

Recording Results.

 Fluid Friction

Take a piece of paper and throw it in any direction. It will not go very far. Then fold it in the form of a rocket or a plane. Now it can fly in the desired direction, at a higher speed. Do you know why?

Home Experiment  4

Aim:  ...To compare fluid resistance in different media.


What you need:-

A few tall identical bottles.

A few drawing pins.

Take a few tall bottles. Fill one with water and keep the other empty so that it will contain air. 

Now place the pins on a board as shown and drop them at once by tilting it.

Note the order of them reaching the bottom.

Cause of Fluid Friction

Any object moving in a fluid will have to clear the molecules in the medium to obtain a path. The fluid friction results from the collision of these molecules with the moving object. In the case of a liquid, there will be the inter – molecular attractions, which may have to be broken by the moving body.

In gases there are no such molecular attractions. Therefore the friction in a gas must be much less.

Fig. 9. Invisible air particles obstructing a rider.

Fig.10, Alis's fall is reduced by fluid friction


Fig. 11.  Better streamlined than the best submarine

Man is not the first animal that has designed body shapes to reduce fluid friction. Think of the fast swimming fish like tunas and sharks or the birds like the hawk and king fishers.

In the past, the vehicles were not streamlined. There were mudguards, lamps and door handles that were projecting out. Nowadays almost all fast moving vehicles are streamlined.

Comparing Sliding Friction and Fluid friction


 Sliding Friction

Fluid Friction


Produced by rubbing of surfaces.

Produced when moving inside a medium such as air or water. Mainly due to the collision of molecules of the medium, on the front area of the body.

Effect on speed:-

There is a maximum value. Does not increase with speed.

There is no limit. Increases with speed.

Method to reduce:-

Can be changed by altering the nature of the surfaces.

May be reduced by streamlining.

Often reduced by lubricants, bearing or wheels.

Smooth surfaces reduce especially in water.

Method to increase

Having treads as in Car Tyres.

Large surface area as in Parachutes.




Changing Friction.

Reduce Friction

Increase Friction

Making the surface smooth and hard

Making the surface rough and soft(like rubber)

Reduce the tightness (The Reaction force)

Increase the tightness.

Use a solid, liquid or a gaseous lubricant.

Remove particles between surfaces.

Use roller or ball bearings.

Use materials such as rubber.

Use a streamlined body shape.

Exposing a bigger frontal area.

An Interesting Project 

Collect pictures of classic cars, arrange them according to their year of production and study how the designs have improved to reduce air friction.


Q. 1.0   A wooden block is kept on the table.

1.1   Name the numbered forces.q 1.jpg

1.2   What can you say about the values of 1 and 3.

1.3  If you exert a pulling force of 3 N by the force measurer and the block is not moving, what is the friction?

  1. 0  , b.  3 N, c. More than 3N. d. Less than 3N.

1.4  When the pull is increased to 4N, the block started moving. What will be the friction if the pull is increased to 5N?

  1. 0.    , b.   5N,  c. 4N,  d. 3N.                                         (4x5 =20 marks)

Q 2.0.  Answer the questions in respect of this figure.


2.1 Name 2 parts of this machine that could cause air friction.

2.2 Name 2 parts where friction has to be reduced.

2.3 Where do you think ball bearings have been used to reduce friction?

2.4 Mention 2 parts where friction is essential .

2.5 Why do tyres have treads and mention 2 ways  how they can be a help to  the rider.

(4x5 =20 marks)


Q. 3.0  John is making a great effort to pull the load.John.jpg

3.1  At which point should he increase friction?

  1. Floor,  b. shoes, c. grip and shoes. d. floor, shoes and grip.

3.2  Where should he reduce friction?

  1. Floor,  b. shoes, c. grip and shoes. d. floor, shoes and grip

3.3  Give a method by which friction may be altered according to requirement at each of these places.     i. on the floor below the load.

                          ii.  The grip between rope and palm.

                          iii,   Between shoes and floor.                          

                                                                                                                                         ( 5x4=20 marks)

Q. 4.0

Consider this to be a moving vehicle and answer the questions.

4.1   The Direction of unwanted friction?         a. Up  b. down.  c. right  d. left.

4.2 What causes the un-wanted friction?  a. ground. b. drag. c. tyres.  d. weight.

4.3   Which will reduce this?  a. Speeding.  b.streamlining.  c. reducing weight. d. treads in tyre..                                                                        (3x5 =15)


There are 4 moving objects here. Which of these actions will help to make them

move easily.

X.   Reduce         Y. Increase.     Z. No change..

Use the correct letter against the red numbers..


Contact friction.


Fluid friction.




  1. .  







  1.  .









Q. 6.0

6.1  What is coefficient of friction?

                                                  (1 + 4 = 5 marks)

For Answers click Answers