Newton’s Laws of motion

“FIRST LAW OF MOTION”

STATEMENT:

“A body continues its state of rest or of uniform motion in a straight line unless an external force acts on it”.

EXAMPLE:

• A book at rest on table will remains in its state unless someone from outside changes its position.

• A ball rolling on floor will keep moving unless some external friction force stops it’s motion.

EXPLANATION:

Newton’s first law of motion consisted of two parts. The first part explains us the definition of force and the other part introduces inertia. Hence, Newton's first law is also called law of inertia.

INERETIA:

“Inertia is the property of an object due to which it tends to continue its state of rest or uniform motion. Inertia is resistance to change the state”.

EXAMPLE OF INERTIA:

• When a bus starts moving the passengers feel a backward jerk, because their lower part of body moves along the motion of bus but the upper part of the body tends to stay at its initial position of rest.

• When you stop paddling your bicycle it does not stop at once. The bicycle continues moving for some time till the friction overcomes it.

“SECOND LAW OF MOTION”

STATEMENT:

Newton's second law of motion describes the relation between force and acceleration. Newton's second law of motion states that: ”when a net force acts on a body it produces acceleration in the direction of force. The acceleration is directly proportional to force and inversely proportional to mass of body”.

EXPLANATION:

Consider an object of mass $m$. Let a force of magnitude $F$ being applied on this object, then according to Newton’s Second Law of Motion, acceleration of magnitude $a$ will be produced in the object in the direction of force applied. Further we can conclude that,

1. The acceleration produced in an object is directly proportional to the magnitude of applied force i.e.

$a \propto F$

2. Also, the acceleration produced in an object is inversely proportional to the mass of that object, i.e.

$a \propto \frac{1}{m}$

Combining the above two equation, we can write

$a\propto \frac{F}{m}$

$a = (constant) \frac{F}{m}$

$a=K \frac{F}{m}$

$FK = ma$

Where, $K$ is the constant of proportionality. In M.K.S system, the value of K is unity. i.e$K = 1$

Therefore, the equation of acceleration can be written as:

$\boxed {F =ma}$

Unit Of Force

F=ma

Unit = $kg×m/s^2$

Unit of Force = $Newton$

“THIRD LAW OF MOTION”

This law describes what happens when a body exerts a force on another body. Newton’s third law of motion can be defined as:

STATEMENT:

“To every action, there is an equal and opposite reaction”.

EXAMPLE:

• In the engine of the rocket, gases formed due to the combustion of fuel, rush out with great speed through jet on the backside of the rocket and as a reaction the rocket moved in the upward direction.

• An object lying on a floor. There will be two forces acting on it, i.e. its weight which is acting downward and the normal reaction by the surface which is due to the reaction to weight of the object acting at the surface of the earth.