Laws of motion

There are three laws of motion

First law of motion

An object that is at rest will remain at rest and an object in motion will remain in motion unless acted upon by an external unbalanced force

Second law of motion

Force is directly proportional to mass and acceleration

This gives the formula:

F=m×a\vec{F} = m \times \vec{a} \newline

Force = mass * acceleration, where force and acceleration are both vector quantities.

Units for force, mass and acceleration

Third law of motion

Arguably the most popular one, this law says:

Every action has an equal and opposite reaction.

However this law is misleading: I had a doubt - If every action has an equal reaction, then how is any movement in the universe possible at all? Shouldn't every force just get cancelled by it's partner force?

Well, it turns out, that the partner forces are acting on different objects. If object a is exerting a force of 10N on b, b is exerting a force of 10N on a. Ok. But what's missing here is that the 10 N on b don't cause the same effect as 10N on a. Since according to law 2, acceleration = force / mass, the lesser the mass, more the acceleration, even if force is same.

Every force has a partner force, and note that the partner force is always opposite. If a force is acting from A on B, then it's partner force will always be acting from B on A.

Clarification on partner forces

This drawing contains a table, denoted by T, an object, denoted by A, and the earth, denoted by E. Every force is in the form of FXYF_{XY}, where F is the force, X is the object that the force is being applied on, and Y is the object applying the force


Partner Force

AE (Force exerted on the object by the earth, a.k.a Gravity)

EA (Force exerted on the earth by the object)

AT (Force exerted on the object by the table)

TA (Force exerted on the table by the object)

Note that all forces do not have to be pushes. AE and EA are examples of pull forces, while AT and TA are examples of push forces