### Force definition in Physics

##### March 28, 2017

# Force definition in physics

Force is a push or pull that causes change of velocity of an object i.e. force is a kind of interaction that causes an object with mass to accelerate .

A force is a Vector quantity as it has both magnitude and direction.

This is the *force definition in physics* or *definition of force in physics*.

The SI unit of force is *Newton*. *Dyne* is its CGS unit. Force is generally represented by the symbol ** F**.

## Examples of force – force definition

As we have covered the force definition in physics now its time to mention a few *common examples of force*.

If you are pushing a box, you are certainly applying a force. If you are pulling a rope you are again applying a force.

As a force is applied on a body, it results into an acceleration of the body. Force can be of different types like friction or frictional force, gravitational force, tension in string etc.

## Force causes Acceleration – force definition

Acceleration is the __rate__ of change of __velocity__. Say you are walking with a velocity of 2 meter/sec. Now you speed up little more by applying your **muscle force** and start to walk with a velocity of 3 meter/sec.

So the change of velocity is = (3 – 2) meter/second= 1 meter/second.

And you took, say, 2 seconds to increase your velocity.

So Acceleration = the rate of change of velocity = (Total change of velocity) / (time taken to change the velocity)

= (1 meter/sec)/2 second = (½) meter/second^{2 }=0.5 meter/**second**^{2}.

Note that the acceleration has ‘time’ ie **second**** twice** in its unit and hopefully you have noticed how it got that twice in the above calculation.

## Force Formula / Force equation

It can be proved from **Newton’s 2nd Law **of motion that, **Force(F) = Mass X Acceleration**

[ see the proof here: Newton’s 2nd Law and equation of force ]**i.e. F=m * a, (here m=mass and a =acceleration)**

If we kick a ball at rest, it starts moving. That means a change in its velocity happens and it took some time for this change. That means an acceleration of the ball is generated by the kick (a push). That’s an example of a force which generates an acceleration of the body on which the force is applied.

Similarly if we kick a moving ball and its velocity changes (change in magnitude or direction or both of velocity) then it’s because of the force applied through the kick.

And if we stop a running ball, similarly we are applying a force, as its velocity of the ball changes in a given time.

## What is Net Force? – force definition

Force is a Vector Quantity as we need to mention both the magnitude and direction of a force to define it.

And Net force on a system is the **vector summation** of all components of all forces acting on a system.

This is also called the **Resultant force.**

**No Net force, No acceleration – Newton’s first law of motion**

If there is no net force acting on a system i.e. if net force is 0 then from the formula of Force above, acceleration is also 0. (considering a non zero mass).

[ F= 0 , hence F = m a = 0. Cnsidering, mass m is non-zero, so obviously a = 0 ]

Note that we are saying ‘no net force” that means this is true even when there are multiple components of force working in or on a system but eventually they nullify after vector summation and net effort or force on the system is zero. This system where all forces got balanced and the net force is zero that system is known as *balanced system*. Similarly, an *unbalanced system* has a *net force* working on it.

## Newton’s first law from Newton’s second law – force definition

As example, say two teams in a tug of war are pulling each other in opposite direction with equal forces. In this case the net force on the rope is zero.

In details using equation of force derived from **Newton’s second law**,

F= m * a;

when net force (F) is zero then we can write,

0=m * a

or, a=0 (considering mass m is non-zero)

So when net force is zero on a body or a system, then acceleration of the body is also zero. *This is exactly what is expressed by the First Law of Motion. *

### Newton’s First Law of motion – explanation

If no net force is applied externally on an object, a stationary object will not have any acceleration i.e. the static object will continue to remain static.

Similarly if no net force is applied on a moving body, it won’t have any acceleration i.e. it will continue to move with uniform velocity.

This is said by the Newton’s First Law of motion and true for all inertial frame of reference.

Read this to know about** Inertial frame of reference.**

## Unit of Force – Newton

We mentioned about the units of force when we discussed the force definition in physics. Lets revise once more.

In SI the unit of force is** Newton**.

**Newton**: One Newton is the amount of force which can generate an acceleration of 1 meter/second^2 when applied on a mass of 1 Kg.

## Force cgs unit – Dyne

CGS unit of force is Dyne.

**Dyne**: One Dyne is the amount of force which can generate an acceleration of 1 centimeter/second^2 when applied on a mass of 1 gm.

### Relation between Newton and Dyne

1 Newton = 10^5 Dyne

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