Loading....
Coupon Accepted Successfully!

 

Newton's First Law of Motion: The Law of Inertia

Newton’s first law of motion states that "every body continues in its state of rest or of uniform motion in a straight line unless it is compelled to change that state by an external unbalanced force."
 

Interpretation of First Law

The first part of the law, namely, "every body continues in its state of rest unless it is compelled to change that state by an external unbalanced force", is a common experience.

If we place a book on a table, it stays there unless someone removes it, i.e. applies a force on it.

The second part of the law, namely, "every body continues in its state of uniform motion in a straight line unless it is compelled to change that state by an external unbalanced fore" is more difficult to understand.

The closest approximation to this ideal situation is motion of a star in free space. Observations taken over many decades show that the motion of the stars is uniform and will continue to be so.

A stone rolled along a road comes to rest after going only a short distance. If the stone is replaced with a smoother body, like a ball bearing on a long sheet of glass. If we roll the ball bearing on the glass sheet with the same push or force, it travels a much longer distance before coming to rest.

If this experiment is done in a chamber from which air has been removed, we should expect the ball to go still further. We are trying to reduce friction which offers resistance to motion. If the friction is completely absent, then a body in a state of uniform motion must continue in that state for all time. It should never come to rest. In other words, the speed of a body can be changed only if an external force acts on it.

The phrase "in a straight line", denotes that the direction as well as the speed of a moving body will remain the same (i.e. the velocity remains the same) unless an external force acts on it.

Hence, from the first law, force is qualitatively defined as something which, while acting on a body, changes or tends to change its state of rest or of uniform motion.

Newton’s first law of motion tells us that, if no net external force is exerted on a body, it will continue to stay at rest (if it is already at rest) or continue to move with a constant velocity (if it is already moving). If the net external force on a body is zero, its acceleration is zero. Acceleration can be non-zero only if there is a net external force on the body.
 




Test Your Skills Now!
Take a Quiz now
Reviewer Name