Newton’s First Law of Motion

To study Newton’s first law of motion, the concept of equilibrium should be clear to us. Whenever a number of forces act on a body and they neutralize each other’s effect, the body is said to be in equilibrium. In such a case there is no change in the state of rest or of motion. If however, the system of forces have a resultant, the state of rest or that of motion undergoes a change. This is explained by Newton’s first law of motion. 

It states that,” Every body continues in its state of rest or of uniform motion in a straight line unless it is compelled by some external force to change that state. Therefore, every object persists in its natural state of motion i.e. continues to be at rest or moves in a straight line with uniform (constant) velocity, in the absence of a net external force acting (impressed) on it.

It can be easily deduced from the statement of change in the state of motion. It is directly related to a frame of reference about which we have discussed earlier. To mark the point here, we can discover that by viewing objects from different frame of references the natural state of motion as perceived by different observers will be obviously different (can only be same if the frames are truly equivalent). Therefore, the change in state will also depend on the choice of reference frame. Finally, the amount of acceleration produced in a body (or change in velocity) will depend on our choice of reference frames.

Law of Inertia

Inertia is the property of all bodies by virtue of which they are unable to change their state of rest or of uniform motion in a straight line without the help of an external force. In other words inertia can also be termed as a resistance to change the state of motion of a body.

Inertia can be classified into following three categories.

(a) Inertia of Rest:-

It is the property of a body by virtue of which it is unable to change its state of rest without the help of an external force. 

(b) Inertia of Motion:-

It is the property of a body by virtue of which it is not able to change its speed without the help of an external force. 

(c) Inertia of Direction:-

It is the property of a body by virtue of which it is unable to change its direction of motion without the help of an external force. 

Qualitative definition of force from first law:-

Newton’s first law states that there cannot be any change in the state of rest or that of motion of a body unless some external force acts upon it. In other words force is an agent which is capable of producing any change in state of rest or that of motion (including direction). This provides a qualitative definition of force.  

Some Conceptual Questions

Question 1:-

A car moving at constant speed is suddenly braked. The occupants, all wearing seat belts, are thrown forward. The instant the car stops, however, the occupants are all jerked backward. Why? Is it  possible to stop an automobile without this ‘jerk’? 

Solution:-

Newton’s first law states that, without any external force, if a body is at rest, it will remain at rest and if the body is moving with constant velocity, it will continue to do so. When the car is suddenly braked, due to the inertia, the occupants in the car will tend to move in the forward direction of car. When the car stops the sit belt in the car will produce backward momentum on the occupants. Since the all the occupants wearing seat belts, therefore the occupants are all jerked backward.

Yes, it is possible to stop an automobile without this jerk. This can be done by slowing down the car a little longer time.

Question 2:-

Why do you fall forward when a moving bus decelerates to a stop and fall backward when it accelerates from rest? Subway standees often find it convenient to face the side of the car when the train is starting or stopping and to face the front or rear when it is running at constant speed. Why?

Solution:-

Newton’s first law states that, without any external force, if a body is at rest, it will remain at rest and if the body is moving with constant velocity, it will continue to do so.

When the moving bus decelerates, due to inertia, the body will tend to move in the forward direction of the bus. Therefore, you fall forward when the moving bus decelerates to a stop. Again, when the bus accelerates from rest, due to inertia, the body will tend to maintain the rest position. That is why; you fall backward when the bus accelerates from rest.

Question 3:-

A mass 'M' is lying (figure shown below) on a table which is at rest (w.r.t. the table on which it is kept). Explain its state with the help of Newton's First Law of motion.

Solution:-

Since 'M' is lying on a table, there is no external force acting on it (forget about gravity just for the immediate discussion). As per Newton's first law of motion it will keep on lying at rest with respect to table for infinite time.

Here, comes out a very important, intrinsic (that is inherent) property of a body which is that it retains its state of motionlessness (as well as of motion, if it is in motion) which is termed as INERTIA of an object. This is present in all materialistic bodies in this universe.

Related Resources

• Click here for the Detailed Syllabus of IIT JEE Physics.

• Look into the Sample Papers of Previous Years to get a hint of the kinds of questions asked in the exam.

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