5. Collect at least six objects of different materials. Check them for their friction
by sliding an object on their surfaces. Now arrange them according to their friction
(high to low). How their friction can be reduced? Suggest some methods according
to the materials.
(6 marks)
Answers
Answer: Y
ou might have seen a driver of a
car or a truck slowing down the
vehicle at a traffic signal. You, too,
slow down your bicycle whenever
needed by applying brakes. Have you
ever thought why a vehicle slows
down when brakes are applied? Not
only vehicles, any object, moving over
the surface of another object slows
down when no external force is
applied on it. Finally it stops. Have
you not seen a moving ball on the
ground stopping after some time?
Why do we slip when we step on a
banana peel (Fig. 12.1)? Why is it
difficult to walk on a smooth and wet
floor?
Fig. 12.1 : A boy falls down when he steps on a
banana peel
12.1 Force of Friction
Activity 12.1
Gently push a book on a table [Fig.
12.2(a)]. You observe that it stops
after moving for some distance.
Repeat this activity pushing the
book from the opposite direction
[Fig. 12.2, (b)]. Does the book stop
this time, too? Can you think of an
explanation? Can we say that a force
must be acting on the book
opposing its motion? This force is
called the force of friction.
Fig. 12.2 (a), (b) : Friction opposes relative
motion between the
surfaces of the book and
the table
(a)
(b)
You will find the answers to such
questions in this chapter.
2018-19
You saw that if you apply the force
along the left, friction acts along the
right. If you apply the force along the
right, the friction acts along the left
direction. In both cases the force opposes
the motion of the book. The force of
friction always opposes the applied force.
In the above activity, the force of
friction acts between the surface of the
book and the surface of the table.
Is the friction the same for all the
sur faces?
Explanation:
Answer:
Properties of Materials
Transparency/Opaque: The amount of light material allow passing through it is transparency of the material. The maximum amount of light to pass through the material, therefore, they are transparent materials. Examples: Plastic, Air, and Glass.
Translucent materials are, which that only allow light to pass through them partially. Examples: Oiled paper, Coloured syrup, and some sheer materials. An opaque object is the materials are, which don’t allow any light to pass through them. Examples: Wood, Cardboard, and Metals
The appearance of the Material: The look, feel, texture in addition to lustre, colour and quality defines the property of appearance of the material.
Soluble/Insoluble: The nature of the material to completely dissolve in water and therefore termed as soluble material. Materials are completely dissolvable in water, therefore, they are soluble materials. Examples: Lemon juice, Sugar and Salt. materials don’t completely dissolve in water, they form a thin film over the surface of the water, therefore, they are iron rods and copper wires.Examples: Oil, Kerosene and Sawdust.
Float/Sink: weight of the material helps to define this property. The material is lightweight it will float over the surface, therefore, they are floating material. Examples: Insoluble materials like Sawdust, Oil, Plastic and Wood. The material is heavyweight it will sink in water, therefore, they are sinking materials. Examples: Stone and Metals
Heat and electricity conductivity: Materials that allow heat and electricity pass through the material and hence called good conductors of heat and electricity. Good conductor of heat and electricity are which allow the head and electricity pass through them.Examples: Iron rods and copper wires. Bad conductors are the materials which don’t allow heat and electricity pass through them. Examples: hydrochloric acid.
State of a material: Compactness and the consistency of the materials describe the state of a material. There are 3 states in which a material can be classified and they are as solid, liquid and gaseous. Solid materials are the most compact and the particles are densely packed.
Examples: Wood, Paper and Glass. Liquid materials are less compact and particles are moderately densely packed. Examples: Water, Oil and Kerosene. Gaseous materials are least compact and particles are loosely packed. Examples: Air.