PHYSICAL AND CHEMICAL

                       CHANGES

                     FRICTION

Friction :- It is the force that opposes the motion of an object.

Factors affecting friction : -
(i) it is caused by the irregularities on the two surfaces in contact.
(ii) When the surfaces on any two objects are in contact, these tiny hills and grooves  entangle or interlock with each other opposite motion. When we attempt to move any one object, we have to apply force to overcome interlocking.

* Friction increases as the mass increases.

* Friction produce heat.

The factor of friction depends on the :-
• Nature of surfaces in contact.
• Mass of objects in contact.

TYPES OF FRICTION 

(i) Static friction :- Static friction is the frictional force that exists between two bodies as long as they are relatively at rest,  even though an external force is acting upon them.

(ii) Sliding friction :- The force of friction acting between two bodies when they slide  on another with a uniform speed is called sliding friction.

(iii) Rolling friction :- The force of friction that exists between two surfaces when a body rolls over the other body is cold rolling friction.

FLUID FRICTION
• The frictional force exerted by fluids is called drag.
• This drag depends upon the shape, material and the speed of the object.
• Fluids friction is minimum for objects that are narrow in front and broad at the back. This is called a streamlined shape.
Example - Boats and Ships have streamlined bodies to help them move through water easily.
* The substances which reduce friction is call lubricants.

ADVANTAGES OF FRICTION
Friction plays an important role in our daily life. Some examples to prove this are given below : -
• Friction between our feet or foot wear and the ground helps us to work without sleeping. That is why it is difficult to walk on Ice or on a smooth polished floor.
• The friction between the treaded tires and the rough surface of the road enables vehicles to move safely on the road. That is why we observe more incidents of vehicles skidding on greasy or wet roads on rainy days.
• Friction between the brake shoe and the wheel slows down or stops a  moving vehicle.
• We are able to light a matchstick, sew clothes, tie a knot or fix a nail on the wall because of friction.
• Friction between a pencil or a pen and paper helps us to write. That is why we find it difficult to write on smooth surfaces like plastic, glass, etc. It is friction that allows your teacher to write on the blackboard.

DISADVANTAGES OF FICTION
The advantages of friction are numerous. But it has some disadvantages too:-
• Friction wears out the rubbing surfaces like screws and ball bearing of machines or soles of shoes.
• The objects in contact require more force to overcome friction between their rubbing surfaces. This results in wastage of energy.
• You already knowo that friction generates heat. This heat raises the temperature of a machine, sometimes causing damage to its parts. Hence, proper arrangements have to be made to keep the machine cool. For example, insulation of windings of machines like mixer grinders, hair dryers, etc. may get spoiled due to friction and the motor may burn.

INCREASING FRICTION
We use different techniques to increse friction : -
• Roughen the surfaces
• Dry the surfaces
• Use brake pads

REDUCING FRICTION 

In some situations however, friction is undesirable and we minimize it by using different methods : -
• Polishing the surface
• Lubricating the surfaces
• Using wheels and ball bearing
•Streamlining

                    What is Force?

Force :- Push or pull that affects an object in some way is called a force.
• Force is measurable quantity.
• SI unit of the magnitude of force is Newton (N).
• Force has both magnitude and direction.
[Vector quantity].

EFFECT OF FORCE :-
(i) Force can make a stationary object move.
(ii)  Force can stop a moving object.
(iii) Force can change the speed of a moving object.
(iv) Force can change the direction of moving object.
(v) Force can change the shape of an object.

THERE ARE TWO TYPES OF FORCE :-
(1) Contact force.
(2) Non-contact force.

1. Contact force - Force which require physical contact is known as contact force.
*There are three types of contact force :-
(i) Muscular force
(ii) Mechanical force
(iii) Frictional force

Muscular force - A force that requires that action of Muscles of our body is called muscular force.

Mechanical force - The force generate by a machine (for example the engine of a car) is called mechanical force.

Frictional force - [oppose relative motion]
Force that opposes motion whom two surfaces in contact move or tend to move with respect to each other is called Frictional force.
-  The direction of this force is opposite to the direction of motion.
- It has internal force
- It always generate in pair.

2. Non-contact force - Force that arise due to interaction between objects that are not in physical contact are called Non-contact force.
- Also known as distance force.
- Non-contact forces act From a distance.

* There are three types of Non-contact force:-
(i) Gravitational force
(ii) Magnetic force
(iii) Electrostatic force

Gravitational force - Every object is pulling every other object towards itself. This force of attraction called gravitational force.
- Force acts between two bodies.
- Always attractive.
- Internal force.
- Central force.

• Gravitational force = Gm1m2/r2

Magnetic force - Magnetic force is a force that attracts certain metal objects (like iron and iron fillings) towards a magnet is called magnetic force.
- Magnetic force can be both attractive and repulsive.
• Like pole repel each other.
• Unlike pole attract each other.

Electrostatic force - Force between the  two charged bodies exert a force on other changed or uncharged bodies is called electrostatic force.
- This force deos not require physical contact between the two bodies.
- Non-contact force.
- Central force.
- Electrostatic force could be both attractive and repulsive.

• Two charged bodies repel ech other.
• Two uncharged bodies attract each other.

There are two types of charge:-
(i) Positive charge (+).
(ii) Negative charge ( - ).

BALANCE FORCE :-
Two or more forces acting on an object are said to be balanced forces, if the net affect produced by all the object is zero (0).
- The balanced forces do not change the state of the body.

UNBALANCED FORCE :-
If the net affect produced by two or more forces on an object is not zero, then the forces are said to be unbalanced forces.
- The unbalanced forces change the state of the body.