Let’s say that you want to roll a ball on a very long horizontal surface with constant velocity. Is force required to apply on the ball throughout the motion? You might say yes, as because of external opposing forces such as friction or viscous force the velocity of the ball will keep decreasing and after some time the ball will come to rest. Now let’s consider an ideal situation such that the surface is frictionless and there are no opposing forces on the ball. In that case the ball will continue to move with its constant velocity for infinite time. This situation is explained by Newton’s first law of motion. In this article let’s understand this law in detail.
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In the movies, you must have observed that if a person jumps from a moving car he/she will always fall in the direction of the car. This situation is explained by Newton's first law of motion.
It states that in an inertial frame of reference a body remains in the state of rest or uniform motion in a straight line unless and until an external force acts on it.
In the figure below, the resultant force is zero, hence the object remains unaccelerated.
We can write net force on an object as,
In simple words Newton’s 1st law of motion says that a body will not start moving until and unless we apply external force on it. If the body is in motion, it will not stop or change its velocity until and unless some external force is applied.
The resistance of a particle to change its state of rest or of uniform motion along a straight line is called inertia.
Note: First law is also known as the law of inertia.
Physical Quantification of Inertia
For translational motion, the body having more mass will have more inertia. Hence, mass is said to be a measure of inertia.
There are three types of inertia:
It is the inability of a body by virtue of which it can not move by itself. A body at rest remains at rest and can not start moving on its own due to inertia.
It is the inability of a body that is in motion to stop by itself. A body in uniform motion does not get accelerated or retarded on its own. It also can not come to rest on its own.
It is the inability of the body by virtue of which it can not change its direction of motion by itself.
Question 1. Which of the following has higher inertia: Eraser or Notebook?
Answer: Notebook will have higher inertia than eraser because notebook has more mass than eraser.
Question 2. Is an external force necessary to keep a body in uniform motion? Justify your answer.
Answer: External force is not at all necessary to keep the body in uniform motion. According to Newton’s second law a body which is in uniform motion will continue to move in uniform motion unless unbalanced force acts on it.
Question 3. A notebook lying on a desk doesn't start moving without an external force, why?
Answer: Because of the inertia of the rest of a notebook lying on a desk it remains at rest.
Question 4.What is physical quantification of inertia?
Answer: For translational motion, the body having more mass will have more inertia. Therefore, mass is a measure of inertia.
Question 1. What is another name for Newton's first law of motion ?
Answer: Newton's first law of motion is also called the law of inertia.
Question 2. The only way to speed up a moving object is to apply a net force to it. [True/False]
Answer: True. According to Newton's first law of motion we can slow down a moving object only by applying force in the opposite direction.
Question 3. What is Inertia?
Answer: The resistance of a particle to change its state of rest or of uniform motion along a straight line is called inertia.
Question 4. What are the types of inertia?
Answer: There are three types of inertia : inertia of rest, inertia of motion, inertia of direction.
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