Force:. When we think of a force, we usually imagine push or pull exerted on some object. For instance,you exert a force on a ball when you throw it or kick it. What happen to an object when it is acted by a force ? Force is commonly defined as a push or a pull. We can apply the forces through contractions or extensions of our muscles .when you open a door, you pull it. By the contractions of your muscles we can exert a force . A push force moves something away from you and the pull force moves something closer. You can push hard or gently ,left or right so the force has the magnitude as well as direction. Force is a vector quantity . The symbol F is used to represent the vactor force . If you pull a spring or kick a ball ,they change their position due to the application of force. When you throw a ball in air it comes back due to the force of gravity of the earth. Force can be categorized into two types one is contact force and the other is the non- contact force. In fact when the objects are in contact directly with the forces are called contact forces. Examples from our daily life are normal push and pull force, surface tension of the liquids, air resistance carried by air planes and frictional force ( the cause of motion) .the second types of force are those which reason to attract and repel without get in touch with an object are called non - contact force. Examples of the non- contact forces are electric force, gravitational forces and magnetic force. These forces act due to the presence of their respective field,s i.e .magnetic field created by the magnetic materials ,similarly the gravitational force is the result of the gravitational field created by the earth itself.
How many common push and pull forces do you apply in daily routine? An object that experience a push and pull has a force exerted on it. Notice that it is the object that is considered to be the system. The world around the object that exerts the force on object is called the environment.Each force has a specific, identifiable cause which is called agent. You should be able to name the agent of each force ,for example when you push a book by your hand then the agent of pushing a book is your hand. The first step in solving such problems is to create a pictorial model. To represent force acting on ball hanging by a rope, sketch the situation .circle the system and identify where the system touches the environment . Next replace the object by a dot and each force is representing by arrows that points the direction of the force.Finally label each force acting on the system. Given below is the figure in which a ball is hanging by a rope. The hanging ball with rope is the system . The system (ball) is represented by a dot in the pictrial model, where as the forces are represented by the arrows giving the directions of the force.
A pictorial model of ball hanging by a rope. You can push or pull hard or gently, left or right ,so force has both magnitude and the direction. Force is a vector quantity. Here are some forces listed with their definitions and symbols.
Different forces ,their symbols ,definitions and directions .
Newton ,s First Law of Motion:. Is there any motion of an object when there is no force acting upon it ? Think of a ball rolling on a surface,how long will the ball continue to roll? The motion of an object depends upon the force and also the quality of the surface. If you roll it on a thick carpet or sand, it will quickly come to rest. If you roll it on a marble ,floor, the ball rolls it for a long time .you could imagine that if all friction eliminated ,the ball might roll forever . Galileo did all the above mentioned experiments and concluded that in ideal case, horizontal motion is eternal .He was the first to recognize the general principles of motion. Newton generalized Galileo ,s results to motion in any direction. According to Newton: by external force we mean any force that results from the interaction between the object and it's environment. In simpler terms, Newton ,s First Law states that when the net external force on an object is zero ,it's acceleration is zero.
Balance and Unbalance Forces:. 
We have have learnt in the above section Newton,s First Law of Motion. It actually predicts the behavior of objects that if the force are belanced in the state of rest such that a man is standing on the floor. The weight is a force which is acting downward and the floor also exerting the force in the upward direction. If both the forces are balanced then the man is said to be stabled or in the equilibrium position. The force of gravity applies the force which is actually the weight of the man. Force shown on a person standing on the floor in a balance position. Similarly the first law describes that the objects in constant motion will continue to move with constant velocity. A car is moving with a constant velocity on the road then the situation is said to be balanced because the car is applying a force by overcoming the friction of the road. The applying force is the power generated by the car whereas the friction of road is the reactive force. The car has applied more force and the friction it will have on less, so the apply force is more as compare to the friction of the road .
Car is accelerating, example of the un- balance force.
Inertia:. The law of inertia is actually called the Newton,s First Law of motion. If an object is at rest it will remain at rest until and unless a force is acted upon it.whereas, if an object is moving with constant speed then it will try to move with the same speed constantantly ,until some force acts upon it and stops it's motion. Newton,s First Law of motion describes that a force is not desirable to keep an object moving because if an object is pushed by some force than it should continue to move unless some force or agent cause it to stop . Apply little push on the paper by the finger in the right hand direction. The paper flew away while the coin drops in the glass.
Car is accelerating ,example of the Unbalance force.
Newton ,s Second Law of Motion: Newton,s First Law describes that what happens to an object when the net force acting on it is zero the object either remain at rest or move with constant velocity. Newton,s Second Law describe what happen to an object that has a non - zero net force acting on it . Newton,s Second Law explains the inter connectivity between the net force applied on an object and it's acceleration. The Second Law discovers the root cause of a change in velocity and the resulting displacement. By the application of force the objects moves, the rate of change of displacement is called velocity and rate of change of velocity is called acceleration.
Force of Friction:. When a body is in motion either on a surface or through a fluid ( air or water) ,there is a resistance to the motion because the body interacts with the surroundings. We can call such resistance the force of Friction. Force of friction is very important in our daily lives. They allow us to walk, run and necessary for the motion of the wheeled vehicles. Consider a block on a horizontal table .If we apply an external horizontal force F on the block, acting to the right, the block remains stationary provided the force is small. The applied force F keeps the block moving and the force which is acting in the left direction is called the force of static friction Fs . As long as the block is not moving we can say that Fs= F. The frictional force is a complicated phenomenon because it involves the study of the surface of bodies at the microscopic level.
The direction of the force of friction Fs between a block and a horizontal surface is opposite the direction of the applied force.
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