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. It will not be conflicting to say that force exists when change is present. There are many forces acting around us which could be immense force like gravitational pull of stars, asteroids falling on earth due to the pull or lesser force like nuclear force, interaction between atoms etc.

Probably this question is simple to answer. The force is more often used when something is done as it was insisted to do so. Same is true in physics. It is the external agency acting on the body that changes the state of the body (either rest or motion). It is expressed in Newtons (N).
Examples of Force:
1) The cyclist in motion needs to apply brakes in order to bring his bicycle at rest. (Here the cyclist is applying external force as brakes)
2) A car which is in rest needs to be accelerated to flinch it or bring it in motion. (Here accelerating is the external force acting)
Force acts as external agency that alters the state of the body. Sir Isaac Newton related the force and motion of a body. If F is the force acting on the body then it can be expressed as the product of mass m and acceleration a. Then the Force formula is given as
The force is a vector quantity. Hence to change the direction of motion a force has to be applied. Applying time t on both the sides
Momentum p gives the cross product of mass and velocity. So
Force in terms of momentum is given as
Force in terms of momentum is given as
Which is called as ForceMomentum formula.
The SI unit of force is newton (N). It is the force that we need to accelerate a kg of mass at a rate of one meter per Second Square which is equal to kgms2 called as Newton (N). The other units of force are dyne, kilogram force (kgf), poundal force (lbf) etc.
How much newton is equal to 1 dyne?
Newton is the force that accelerates a kg of mass at a rate of 1 ms^{2}. Hence
Dyne is a common unit used in CGS system. It is a force that produces an acceleration of 1 cm/s^{2} for a body of mass 1 gm.
Hence the newton and dyne are related as
There are many types of force existing in nature like gravitational force, frictional force, nuclear force, electrical force, magnetic force, tension force etc. These forces are classified into four types namely
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