Laws of Motion // Newton's Law of Motion // Newton's First Law of Motion // Linear Momentum // Inertia of Rest // Inertia of Motion

1. Introduction

In this post, we will know about the case of the motion due to the force. Force is the central subject in the mechanics. From everyday  life experience we define the force as push or pull. One of the uses of the force is to produce motion to a rest body or to bring at rest to a moving body.
Figure: Laws of Motion 

In the case, if all the particles of the object move in the same way, the motion is translational and the object is called as point object. If the particles of the object has different linear displacement but same angular displacement, rotational motion takes place. Consider for example, a rolling ball. In this section we talk about the basic laws of motion due to the force. Newton worked about motion scientifically and concluded some results which are now called Newton's laws of motion. According to Newton there are three laws of motion. These three laws suggest force in different ways. First law defines force qualitatively, second law defines quantitatively and third law says that single and isolated force is not possible.
A force may cause both the translational as well as rotational motion on an object. 

2. Newton's Laws of Motion

Newton's First Law of Motion:

Newton's first law says, every body in this universe continues in its state of rest or of uniform motion in a straight line unless it is compelled by some external force to change the state.
According to the above statement we conclude that to change the state of rest or motion, an external attempt is required which is called the force. Internal force does not produce motion in physical objects. Therefore the 1st law gives the definition of force. The first law is also called the law of inertia. According to 1st law, a body at rest remains at rest or moves
 with constant speed on a straight line if no external force acts on it. This means, a body cannot change in state of rest or motion by its own desire. This inability to change the state of a body is called inertia.

There are two types of inertia (i) inertia of rest and b) inertia of motion:

(i) Inertia of rest: The opposition of a body to change its state of rest is called inertia of rest.
Examples of inertia of rest are as follows:
(a)When a bus suddenly moves, the passenger falls backward. It is because the lower part of the body of the passenger in contact to the bus is in motion but upper part tends to be at rest. So the passenger tends to fall backward. 

(b) When we shake a mango orange, the oranges fall down. It is because when we shake the tree, the oranges tend to be at rest due to inertia where as the branches are in the motion. That is why the oranges get detached from the branches.

(c) If we suddenly pull a paper over which a mobile phone is kept, the mobile phone remains at rest. It is because the mobile phone tries to remain at rest. 

(d) When we hit a blanket with a stick, the dust particles come out of it. By hitting the blanket it is set in motion where as the dust particles remain at rest. 

(ii) Inertia of motion: The opposition of a body to change its state of motion is called the inertia of motion,
Examples of motion of inertia can be given as follows:
(a)When someone jumps from a running train, that person fall forward. It is because the foot comes to rest but his/her upper part is still in motion due to inertia and he/she fall forward.

(b) When the brake of the car is suddenly applied, we tend to fall forward. It is because the lower part in contact to the car comes to rest where as the upper part is still in motion. So we fall forward.

(c) An athlete runs a certain distance before taking a long jump. It is because the velocity of the athlete in air is to the velocity at the time of jump and covers a longer distance. 

3. Linear Momentum
Momentum is the motion associated with the physical body. Quantity of motion possessed by a body depends upon both of its mass and velocity. So the product of moss and velocity is the measure of the momentum, that is,

                                                   P = m v.

This is also called linear momentum. It is a vector quantity whose direction is in the direction of the velocity. Its unit is kg m/s in SI-units.
When there is change in the momentum of a body under experiment, there is change in its velocity, i.e. there is acceleration. According to Newton's 1st law of motion, the change in motion is produced by the external agent. So there is a relation between the change in momentum and the force.