Newton’s laws of motion
Newton has given three laws to describe the motion of bodies. These laws are known as Newton’s laws of motion. The Newton’s laws of motion give a precise definition of force and establish a relationship between the force applied on a body and the state of motion acquired by it.
Newton’s first law of motion:- A body at rest will remain at rest, and a body in motion will continue in motion in a straight line with a uniform speed, unless it is compelled by an external force to change its state of rest or of uniform motion.
The tendency of a body to remain at rest (stationary) or, if moving, to continue moving in a straight line, called inertia. Newton’s first law recognizes that every body has some inertia. Inertia is that property of a body due to which it resists a change in its state of rest or of uniform motion. Greater the inertia of a body, greater will be the force required to bring a change in its state of rest or of uniform motion. Mass is a measure of the inertia of a body. If a body has more mass, it has more inertia. That is, heavier objects have more inertia than lighter objects. Example:- a stone has greater inertia than a football. If we kick a stone, it will not move because of its high inertia but if we kick a football, it will move a long way. A stone has more inertia than a football. A cricket ball has more inertia than a rubber ball of the same size. So, the inertia of a body depends on its mass.
So we conclude that to overcome the inertia and make a body move from rest, we must apply an external force.
It should be noted that Newton’s first law of motion is also some times called Galileo’s law of inertia.
Some more examples of law of inertia:-
- When a tree ( having flexible stem) is shaken vigorously, its fruits and leaves fall down. This is due to the fact that because of their inertia.
- When a hanging carpet is beaten with a stick, the dust particles start coming out of it. This is because the force of stick makes the carpet move to-and-fro slightly but the dust particles tend to remain at rest due to their inertia and hence separate from the carpet.
- When a car or bus starts suddenly, the passengers fall backward. This is due to the fact that because of their inertia.
- When a running car or bus stops suddenly, the passengers are jerked forward because due to inertia.
Newton’s first law of motion gives us a definition of force. A force is something which changes or tends to change the state of rest or of uniform motion of a body. Force is a vector quantity.
Momentum:- The momentum of a body is defined as the product of its mass and velocity. The force required to to stop a moving body is also directly proportional to its velocity. Thus, the quantity of motion in a body depends on the mass and velocity of the body.
Thus, Momentum = mass × velocity
P = m × v
where, p = momentum
m = mass of the body
v = velocity of the body
If a body is at rest, its velocity is zero and hence its momentum is also zero. Thus, the total momentum of the gun and bullet before firing is zero because their velocity is zero. momentum is a vector quantity. The SI unit of momentum is kilogram metres per second or Kg.ms-1.
Every moving body possesses momentum. Since momentum depends on the mass and velocity of a body, so a body will have a large momentum : a) If its mass is larger, or (b) If its velocity is large, or (c) If both its mass and velocity are large.
Some examples of momentum:-
- A karate player can break a pile of tiles or a slab of ice with a single blow of his hand.
- A cricket ball is not very heavy but when it is thrown with a high speed, it acquires a very momentum and sometimes hurts the batsman.
- A car or bus may not be running at a high speed but because of its high momentum which may hurt the person coming in its way.
Momentum is considered to be a measure of the quantity of motion of a moving body.
Ex:- What is the momentum of a man of mass 75kg when he walks with a uniform velocity of 2m/s ?
here, mass = 75kg
velocity = 2m/s
We know that, momentum = mass × velocity
p = m × v
p = 75 × 2
p = 150 kg.ms-1.
