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.
 
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.
Results
Reading | Observation | Type of friction. | 1N | No movement | Static | 2N | No movement. | Static | 3N | No movement | Static | 4N | still no movement. | Static | 5N | The body moved ! | Dynamic. |
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.
Analysis
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.
Question
|
Answer
|
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
Analysis
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.
Lubrication
Home
Experiment 3
Aim:- ... Testing Lubricants.
Method:-
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.
Method:
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
|
Creating:-
|
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. 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? 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? 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) (3 Q. 3.0 John is making a great effort to pull the load. 3.1 At which point should he increase friction? Floor, b. shoes, c. grip and shoes. d. floor, shoes and grip.
3.2 Where should he reduce friction? 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) 5.0 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..
| A Contact friction. | B. Fluid friction. | 4.1.

|
Y | .
X | 4.2.
|
X |
X | 4.3. 
| .
X |
| 4.4. |
Y |
Z |
|