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Why Force? We can say it's because it makes the world go round! And without  force we would fall of things, force pushes or pulls us back. When a Force is acted on an object the Motion changes – a push or a pull on an object that can cause it to change direction is termed as Force. Force only exists as a result of an interaction.

Let’s take an example from our day to day life, when we sit on a chair, we need force to push us and hold us, so we won't fall. 

In this section we will be discussing in detail about Force, different types of Forces and Use of Force.


What is force?

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In physics, a Force is a push or pull upon an object resulting from the object's interaction with another object. Nothing moves, stops, or changes directions without Force. Friction is a force that slows down the motion of moving object. Force is a quantitative interaction between two physical bodies, such as an object and its environment. 

Example: When we push open the door, throwing a ball, pushing a table, lifting a bag we apply Force on it. 

A force results from the interaction between two objects. For a force to accelerate an object it must come from outside it. You can't pull yourself up by your own shoes. Sir Isaac Newton founded a formula connecting the force and the motion of an object in a straight line and it is widely called as Newton’s second law of motion. It states that the sum of the forces F on an object is equal to the mass m of the object multiplied by the acceleration a, of the object.

Algebraically it is stated as, 

F = m $\times$ a 

Force is a vector quantity which has both magnitude and direction. Hence to change the direction of motion of an object even with the same magnitude of velocity a force has to be applied. Thus, the greater the mass of the object being accelerated, the greater the amount of force needed to accelerate the object.
A force is measured in terms of the acceleration caused on a particular mass. Forces are measured in different units. Apart from the systems of units, the unit of forces are classified into two systems called :

1) Absolute system
2) Gravitational system

In absolute system, the fundamental units are unit of length, unit of mass and unit of time. In gravitational systems, the fundamental units are unit of length, a unit of force and a unit of time. The measurement of forces in absolute system is not dependent on gravitational force, whereas, measurement of forces in gravitational system is dependent on gravitational forces. Hence, the measure of forces in gravitational system differs on other planets compared to earth.

When a mass of 1 pound is subjected to an acceleration of 1 ft/sec2, the force applied is called 1poundal. The unit is abbreviated as pdl.

A pound force is defined as the gravitational force exerted on 1 pound mass. It is denoted as lbf. When we say that the weight of the person is 150 pounds, we actually mean 150 lbf.

Poundal and pound force are the units of measurement of forces in fps system.

In metric system a force of 1 kilogram force is the force exerted on 1 kilogram mass. It is similar to pound force in fps system. It is denoted as kgf.

In cgs system a force is measured in terms of dynes. A force of 1 dyne produces an acceleration of 1cm/sec2 on a mass of 1 gram.

In SI units, the unit of force is defined as newton. This unit is named in the honor of Sir Isaac Newton who actually had given the definition of a force. One newton produces an acceleration of 1 m/sec2 on  a mass of 1 kilogram. It can easily be derived that 1 newton is equal to 105 dynes. The unit of newton is abbreviated as N.

The gravitational units and absolute units are related by acceleration due to gravity. That is,

1 pound force $\approx$ 32 poundage and 1 kilogram force ≈ 9.81 newtons

Relationship between newton and dyne

One newton is the force that produces an acceleration of 1m/s2 in a body of mass 1kg.

1 newton = 1 kg x 1m/s2

Another such unit for force in the CGS system, is the dyne. One dyne is the force which produces an acceleration of 1cm/s2 in a body of mass 1gram.

1 dyne = 1 gm x 1cm/s2

The relation between one dyne and one Newton is

1 N = 105 Dyne

or 1 Dyne = 10-5 N

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As per the basic definition of force, the fundamental equation of force is,

F = m $\times$ a,
where F is the force, m is the mass of the object on which the force is applied and a is the acceleration generated.

At this stage we will define a quantity called momentum. It is a vector quantity defined as the product of mass of an object and its velocity. It is denoted by the letter p. The equation is,

p = mv

When we multiply  the equation of force by the time t on both sides,

F $\times$ t = m $\times$ a $\times$ t = mv, which we defined as momentum


F $\times$ t = p  or F = $\frac{p}{t}$

When ‘t’ is infinitesimally small, the force becomes large and such force is called impulse force.

Momentum is a conserved quantity. Momentum can not be destroyed, it only gets transferred. This concept helps us on the study of collision of objects.
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We have generally defined force as an external agency that changes the velocity of an object with a specified mass. Let us see in what situations the velocity or the state of objects are changed and thereby classify the types of forces:
  • Contact Force
  • Non Contact Force
  • Tensile Force
  • Normal Force
1) Contact Force :
When a body is moving along the surface of another body, the motion is resisted by a force called frictional force. The frictional force is generated along the contact surface of both the bodies. The magnitude of a frictional force depends upon the nature of the materials of the bodies that are in contact. The next example, which is also encountered by us in real life is the frictional force.

2) Non Contact Force :
The force which exists between two bodies which are not in contact with each other is a Non Contact Force. A very obvious force we see in day to day life is the gravitational force. Any object away from the earth is pulled towards earth by the force called gravitational force. Even if an object is moving upwards, the gravitational force tends to slow down the vertical movement of the object. The specialty of gravitational force is that it is a Non contact Force.

3) Tensile Force :
We already explained that weight of an object is nothing but a type of gravitational force. When an object is suspended with a string, wire or a chain, the force of the weight of the body acting down is countered by an upward force as per Newton’s third law. This force is transmitted to the item used for suspension. Such a force is called Tensile Force. The material of the string, wire or chain should have the capacity to absorb this force, else it will get snapped.

                                                Tensile Force

4) Normal Force :
Even if, an object is not suspended and just simply resting on a surface, the force of the object on the surface is counter acted by a force in the opposite direction. This force acts in the direction that is normal to the surface. This type of force is called as normal force. It may be noted that a frictional force is proportional to the Normal Force.
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More topics in Forces
Unit of Force Equation for Force
Types of Forces Work and Energy
Effects of Forces Deformation of Solids
Universal Forces Force and Pressure
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