Introduction:- if you push a chair, it tend to move. You can push or pull and object, you exert a force on the object. You can push or pull an object in different direction,gently or hard. This means force as a direction and magnitude.

Force is that cause which produce acceleration in the body on which it acts.

       We see that a force or a set of forces acting on a body can do three things.

1. A force or a set of forces can change the speed of a body.

2. A force or a set of forces can change the direction of motion of a body.

3. A set of forces can change the shape of a body.

Resultant force: If a single force acting on a body produces the same acceleration as produced by a number of forces, that single force is called the resultant of this individual forces.

       Resultant force is also called the net force.

Balanced and Unbalanced force: If a set of forces acting on a body produces no acceleration in it, the forces are said to be balanced.

         If it produces a non zero acceleration the forces are said to be unbalanced.

Some common forces:-We have considered example of forces which are exerted by living things. However non living things can also exert forces.

1. Contact forces:- When a body A is in contact with body B then  A can  exert a force on B,and B exert a force on A. These forces are called contact forces.

2. Normal force:- If the contact forces between the two bodies are perpendicular to the surface and contact, the forces are called normal force.

3. Friction:-Two bodies placed in contact can also exert force parallel to the surface in contact. Such a force is called force of friction, frictional force or simply friction.

        Friction is a small if the surfaces are smooth and it is large if surfaces are rough.

4. Forces exerted by a spring:- A spring is made of a coiled metallic wire.

        A compressed spring pushes object attached to its ends.

5. Force exerted by a string:- The magnitude of the  force exerted by a string on the object tied to it is called tension.

6. Weight:- Earth attracts all bodies towards its centre. Started by the Earth and the body is called the weight of the body. Note that this not a contact force.

Unit of force:-

SI unit: Newton (N)

Galileo experiment:- It was Galileo who recognized that a body moves with uniform velocity if no force acts on it.

Newton's first law of motion:-

1. If a body is at rest and no unbalanced force act on it, it remains at rest.

2. If a body is moving and no unbalanced force acts on it, it will continue to move at constant speed in a fixed direction.

3. If an unbalanced force act on a body, the body will accelerate.

         This facts taken together form Galileo's law of inertia or Newton's first law of motion.

The idea was suggested by Galileo and later formulated into a law by Newton.We can state Newton's first law as follow:-

A body at rest will remain at rest and a body in motion will remain in uniform motion unless acted upon by a unbalanced force.

        A body remains  unaccelerated if, and only if,the resultant force on it is zero.

     In such a case the body is said to be in equilibrium.

Inertia:-The inability of a body to change on its own its state of rest or uniform motion is known as inertia.

       Inertia of rest due to which a body at rest remains at rest, and inertia of motion due to which a moving body keeps moving.

Inertia and Mass:- A heavier  body has greater inertia than a lighter body.

Examples of Newton's first law:-inertia of rest, jerks while travelling,the card and coin experiment, striking a pile of carrom coins etc.

Newton's second law:- The magnitude of the net force acting on a body is proportional to the product of the mass of the body and its acceleration. The direction of the force is the same as that of the acceleration.

                      F=kma 

                Where,k=constant.

Linear momentum:- Product of the mass of a body and its velocity is called the linear momentum of the body.

                         p =mv 

Linear momentum also has a direction. At any instant its direction is the same as the direction of the velocity.

Newton 2nd law in terms of Momentum:- It is stated that:- The rate of change of momentum of an object is directly proportional to the net force applied on the object. The direction of the change of momentum is the same as the direction of the net force.

                  F= p2 -p1/t2 -t1.

Definition of 1 Newton:- If a force acting on a body of mass 1kg produces an acceleration of 1  m/s2  in it the force is called 1 Newton.

                          F=ma.

So the net force on a body is equal to the change in its momentum per unit time.t

 Newton's third law of motion:- In any interaction between two bodies, the force applied by the first body on the second body is equal and opposite to the force applied by the second body on the first.

               Such a pair of forces exerted by two bodies on each other is called an action reaction pair.

           Action and reaction are always equal and opposite.

            Action and reaction are two forces that act on two different bodies.

       Any pair of equal and opposite forces is is not an action reaction forces.

Examples:-

        1. Recoil of gun: When a bullet is fired from the gun the gun recoils. This is because the gun exert a force on the bullet in the forward direction, and the bullet exert an equal force on the gun in the backward direction. So the gun moves backward or recoils.

      2. A plane accelerate by rejecting gases in the backward direction.

Acceleration produced by an Action-Reaction pair:- While firing a bullet, a gun exert a force on it. During this time a force of equal magnitude is exerted by the bullet on the gun. Since the bullet has a small mass, its acceleration is high and it was a large velocity. On the other hand, because of the larger mass of the gun, its acceleration is lower, and its recoils with much smaller velocity.

Conservation of Linear Momentum:-If the net external force on a system of a particle is zero, the linear momentum of the system remains constant.

   The linear momentum of the system remains constant , although the linear momentum each of the two particle got changed.