Newton Laws Review

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First law: is the law of inertia. A body at rest remains at rest u less an unbalanced force acts in it.

Now if a ball is rolled down a floor, it stops after some time. Why so?

Because if resistive forces from the floor, the ball stops.

Inertia is the tendency of a body to maintain its state of rest or of motion. More is the mass, more is inertia.

It is also pertinent to discuss the frame of reference or more clearly the inertial frame of reference.

A frame of reference is like the reference point according to which physical quantities are measured.

Newton’s law of motion is valid for the inertial frame of reference.

Second Law:

Acceleration inversely proportions mass and directly proportionates force. The proportionality constant k=1. And so 

F=ma

This is the first definition of Newton’s second law.

Secondly, the rate of change of momentum equals force. This is Newton’s second law in another version.

So v=dx/dt

And F=dP/dt

P=mv

dP/dt= mdx/dt

Third law:

Every action has an equal but opposite reaction.

The greater the inertia, the lesser is the reaction. A massive body for example plate will show smaller reaction compare to a smaller body. 

Third law has two forms:

Strong form of 3rd law:

When line of action of forces is same. For example on a drop in air, weight pulls it Downward, viscous drag force acts upward.

Similarly, for a drop balaced by electric and magnetic field acting in opposite direction but with same reference. 

Weak form of 3rd Law:

Here the line of action of the force is not same. A torque is also produced here. 

Figure sums up the discussion.