What is Force in physics?
What is force in physics? 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. Force can be of different types like friction or frictional force, gravitational force, tension in string etc.
Force causes 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 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/second2 =0.5 meter/second2.
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 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)………………………..(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.
What is Net Force?
Force is a Vector Quantity as we need to mention both the magnitude and direction of a force to define it.
And Net force 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.
Force equation physics – Newton’s first law from Newton’s second law
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.
Newton’s First Law of motion
So if no net 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 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
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.
Note: Force is a Vector Quantity.