FORCE
push or pull on a body is named force. Forces are utilized in our everyday actions like pushing, lifting, pulling, stretching, twisting and pressing. A force can't be seen. A force may be judged only by the consequences which it can produce in several bodies (or objects) around us.
Forces have  both magnitude and direction, making it a vector quantity.
 it's measured in SI unit of Newton and represented by the symbol F.

 Effects of Force

 Force can make a stationary body in motion. for instance a football may be set to be in motion by kicking it, i.e. by applying a force
Force can stop a moving body – for instance by applying brakes, a running cycle or a running vehicle may be stopped.

Force can change the direction of a moving object. For example; by applying force, i.e. by moving handle the direction of a running bicycle may be changed. Similarly by moving steering the direction of a running vehicle is modified.

Force can change the speed of a moving body – By accelerating, the speed of a running vehicle may be increased or by applying brakes the speed of a running vehicle may be decreased.

 Force can change the form and size of an object. For example; by hammering, a block of metal may be became a skinny sheet. By hammering a stone may be broken into pieces.
Balanced and Unbalanced Forces

 Balanced Forces
If the resultant of applied forces is adequate zero, it's called balanced forces. Balanced forces don't cause any change of state of an object. Balanced forces are equal in magnitude and opposite in direction. Balanced forces can change the form and size of an object. for instance - When forces are applied from each side over a balloon, the dimensions and shape of balloon is modified.

 Unbalanced Forces
If the resultant of applied forces are greater than zero the forces are called unbalanced forces. An object in rest may be moved due to applying balanced forces.

 Unbalanced forces can do the following:

 Move a stationary object.
Increase the speed of a moving object.
Decrease the speed of a moving object.
Stop a moving object.
Change the form and size of an object.
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 Some Common Forces

 Muscular Force: The force obtained by the working of form muscles is named muscular force. Ex: Lifting objects, doing exercise etc.
Gravitational Force: The force applied by earth on an object in downward direction is named attraction.
Frictional Force: The force which opposes motion is thought as frictional force. It acts within the direction opposite to the speed of body.
Air Resistance: When an object moves through air, air applies atiny low force in direction opposite to velocity. This force is named air resistance.
Newton’s Laws of Motion

 Newton has given three laws to define the motion of bodies. These laws are called Newton’s laws of motion.

 Newton’s 1st Law

 A body at rest will remain in rest, and a body in motion will continue in motion during a line with uniform speed, unless it's compelled by an external force to vary its state of rest or of uniform motion.

 Mass: Inertia is that property of body thanks to which it resists a change in its state of rest or of uniform motion.

 In this above shown picture the inertia of the coin tries to keep up its state of rest even when the cardboard flows off. The mass of an object could be a measure of its inertia. Its SI unit is kilogram (kg).

 Application of Newton’s 1st Law

 A passenger in bus contains a tendency to stay acquiring a line inside a bus thanks to inertia. When the bus takes turn, body of passenger wants to continue acquiring line. thanks to this, it appears that his body bends outwards.
When we hit a carpet it loses inertia of rest and moves. But the dust in it retains inertia of rest and is left behind. Thus dust and carpet are separated.
When a tree is shaken, it moves to and fro. But fruit remains at rest thanks to its inertia of rest. thanks to this fruit breaks off the tree.

 When a car is braked suddenly, the person bends forward violently thanks to inertia of motion. the person may impinge on parts of car hurting himself. Seatbelt won't let the person bend forward. And thus save them from accident.

 Due to inertia of motion even when the car stops, the baggage on the highest of car has the tendency to maneuver forward. Therefore luggage is tied.
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Momentum: The momentum of a body is defined because the product of its mass and velocity.

 Thus, momentum = mass x velocity

 Or, p = m x v

 Where, p = momentum

 m = mass of the body

 v = velocity of the body

 The SI unit of momentum is kilograms meters per second (kg.m/s)

 Note: The force required to prevent a moving body is directly proportional to its mass and velocity.

Change in momentum

 it's defined because the difference between final momentum and initial momentum. Suppose initial momentum is mu, and final momentum is mv, then

 Change in momentum = mv – mu

 Rate of change of momentum: the speed at which momentum is changing is thought as rate of change of momentum.
Newton’s 2nd Law

The rate of change of momentum of a body is directly proportional to the applied force, and takes place within the direction within which the force acts.

Force = change in momentum / time taken

F = mv - mu / t

= But we all know that

F = m x a

Or Force = Mass x Acceleration

Its SI unit is Newton (N).

Thus, one unit of force is defined because the amount that produces an acceleration of 1 m s-2 in an object of mass 1 kg.

Application of Newton’s 2nd Law:

When we stop the ball gradually, we'd like to use less force. this can be easy and safe. If we stop the ball suddenly, we'd like to use larger force which is difficult and might also injure our hand.
In high jump if surface is difficult, athlete’s body changes velocity in very less time. Large force acts on his body thanks to which he may get injured. If the surfaces are soft, athlete’s body changes velocity in additional time. and fewer force acts on his body which is safe.
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Newton’s 3rd Law

Whenever one body exerts a force on another body, the second body exerts an equal and opposite force to the primary body. Or, to each action there's an equal and opposite reaction.

Action and reaction forces are equal and opposite.

Application of Newton’s 3rd Law:

Gun applies force on bullet thanks to which it moves ahead. By Newton’s 3rd Law, bullet also will apply same force on gun in backward direction. thanks to this force, gun moves back. this can be called recoil of gun. Gun moves back only by bit thanks to its heavy mass.
Hose pipe applies large force on water thanks to which water moves ahead. By Newton’s 3rd Law water applies the identical force on pipe backwards. thanks to this force, pipe can move backwards. To stop it, many folks have to hold it.
Man pushes the boat backwards and by newton’s 3rd law, boat pushes man forward.
Man pushes hot-water heater by applying force. By Newton’s 3rd Law, water applied equal and opposite force on swimmer. thanks to this force man moves ahead.
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Cheetah applies forces on ground in backward direction. By 3rd law, ground applies force equal and opposite on cheetah in forward direction. it's thanks to this force that cheetah moves ahead. For running faster cheetah has to apply more force on ground in forward direction.
Conservation of Momentum

If two or more objects apply force on one another with no external force, their final momentum remains same as initial momentum.

Total momentum before collision = Total momentum after collision

Practical samples of conservation of momentum:

In rocket, fuel is burnt  in which gases are ejected downwards. For conservation of momentum, rocket moves up.
Fuel in aeroplane burns and ejects gases in backward direction. Then by conservation of momentum, plane moves ahead.