Newton's Laws of Motion - First Law


Introduction :

According to Aristotle, a constant continuous force is required to keep a body in uniform motion. It is called Aristotle's Fallacy, and Newton's Laws comes under Aristotle's Fallacy. 

 

The three laws of motion proposed by Newton are three physical laws, which all together form the basis of classical mechanics. The three laws of motion describe the relationship between a body and the forces acting upon it, along with the body's motion corresponding to those forces. To be specific, the first law of motion defines the force qualitatively, the second law of motion gives a quantitative measure of the force, and the third law of motion states that a single isolated force doesn't exist.

 

What is Newton’s First Law of Motion?

Newton's First Law of Motion is also known as Galileo's law of inertia. A body continues to be in its state of rest or uniform motion in a straight line unless compelled by an external force to change its state. The law defines the force and state it as a factor, which can change the state of the object.

 

Inertia is the property of a body due to which it opposes any change in its state. Mass of a body is the measure of its inertia of translational motion. It is difficult to change the state of rest or uniform motion of a body of heavier mass and vice-versa.

(a) Mass of a body is a quantitative or numerical measure of a body's inertia.

(b) Larger the inertia of a body more will be its mass.

 

The inertia of rest : A body cannot change its state of rest by itself.

Example:

(a) When we shake a branch of a mango tree, the mangoes fall.

(b) When a bus or train starts suddenly, the passengers sitting inside tend to fall backward.

(c) When a horse starts suddenly, the rider fall backward.

(d) The dust particles in a blanket fall off when it is beaten with a stick.

(e) A coin is placed on cardboard, and this cardboard is placed over a tumbler, such that the coin is above the mouth of the tumbler. Now, if the cardboard is removed with a sudden jerk, the coin falls into the tumbler.

 

The inertia of motion : A body cannot change its state of uniform motion by itself.

Example:

(a) When a bus or train stops suddenly, the passengers sitting lean forward.

(b) A person who jumps out of a moving train may fall in the forward direction.

(c) A bowler runs the ball before throwing it so that this speed of running gets added to the speed of the ball at the time of the throw.

(d) An athlete runs through a certain distance before taking a long jump because the velocity acquired during the running gets added to the velocity of an athlete at the time of the jump, and hence he can jump over a longer distance.

(e) A ball is thrown in the upward direction by a passenger sitting inside a moving train. The ball will fall:-

  • back to the hands of the passenger, if the train is moving with constant velocity

  • ahead of the passenger, if the train is retarding (slowing down)

  • behind the passenger, if the train is accelerating (speeding up)

 

The inertia of direction : A body cannot change its direction of motion by itself.

Example:

(a) When a straight running car turns sharply, the person sitting inside feels a force radially outwards.

(b) Rotating wheels of vehicle throw out mud, mudguard fitted over the wheels prevent this mud from spreading.

(c) When a knife is pressed against a grinding stone, the sparks produced move in the tangential direction.

 

What is an External Force?

According to Newton's first law of motion, it is the push or the pull which either changes or tends to change the state of rest or uniform motion (constant velocity) of a body is known as a force. An external force is a force originating from outside an object rather than a force internal to an object. For instance, the force of gravity that Earth exerts on the moon is an external force on the moon. However, the force of gravity that the inner core of the moon exerts on the outer crust of the moon is an internal force on the moon. Internal forces within an object can't cause a change in that object's overall motion.

  • Effects of Resultant force:

     (i) It may change the speed of the body.

     (ii) It may change the direction of motion.

     (iii) It may change both the speed and direction of motion.

     (iv) It may change the size or/and the shape of the body.

      (v) It may start a motion in stationary body or it may stop a moving body.

  • Units for measurement of force:

     Absolute units -

     (i) N (M.K.S)

     (ii) dyne (C.G.S)

    Other units-

    (i) kg-wt or kg-f {kf-force}

    (ii) g-wt or g-f

 

Newton’s First Law of Motion Examples in Daily Life

Some daily life examples are as below:

(a) A small coin is put on a card and placed over a glass. When the card is flicked away with the finger, the coin drops into the glass.

(b) Suppose we are sitting in a stationary bus. If it starts moving suddenly, we will feel a jerk in the backward direction. It is because our body is in contact with the seat of the bus, and comes in motion with the motion of the bus, while the upper portion of our body remains at the rest due to inertia, and so we feel a jerk in the backward direction.

(c) Similarly, if we push a ball on the ground, it should continue its uniform motion indefinitely, but it stops after covering a certain distance. As soon as the ball starts moving, a force (force of friction) comes into play, which opposes the motion of the ball.

(d) A book lying on a table can't change its position by itself unless a force is applied to change its position.Newton's Laws of Motion - First Law