### Force – Basic Concepts of Physics

##### March 28, 2017

Force is a push or pull. 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. Now what is acceleration?

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 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 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 twice in its unit and hopefully you have noticed how it got that twice in the above calculation.

It can be proved that,

- Force(F) = Mass X Acceleration

- i.e.

F=m * a, (here m=mass and a= acceleration)………………………..(1)

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.

#### No Net force, No acceleration

If there is no external force on a system i.e. if force is 0 then from the formula of Force above, acceleration is also 0. (considering a non zero mass). This is true even when there are a bunch of forces working in or on a system but eventually they nullify and net effort or force on the system is zero.

In details,

F= m * a;

or 0=m * a, (as F=0 )

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

So if no force is applied externally, a stationary object will not have any acceleration i.e. the static object will continue to remain static.

Similarly if no 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.

**Work done by a force: a Primer – read here**

**Friction and Newton’s 3rd law of motion: read here**

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