Before 1600 century, it was believed that the natural state of the matter is the state of rest.
Galileo, later on, suggested that it is not the property of the object to
stop if it is in the motion but to resist any change in its motion.
Newton later on came up with Newton’s First Law which states that if no
external force is applied to an object and the object is at rest it
will remain at rest and if the object is in motion it will continue to
move with the same velocity.
In simple terms, if the applied force to the object is zero, its
acceleration would be zero and if the object is already in motion, it
will continue to move with constant velocity until some external force
is applied to it to accelerate or retardant. Therefore, the velocity change would be zero and so its acceleration.
Every body continues in its state of rest or of uniform motion in a straight line unless compelled by some external force to act otherwise.
Consider an example, that a book is lying on the table. Force of friction is present between the table and the book. Book is at rest and remains at rest until any force is applied to it. Suppose that force is applied to book in a direction which is opposite to the direction of the force of friction and applied force is greater than the force of friction.
Now, if we apply constant force, the book will move at constant velocity until we push harder and increase the applied force against the external forces acting on the book. When we increase the applied force the book will be accelerated. If the applied force is withdrawn, the book will come to rest and its velocity will become zero after covering some distance. Book will come to rest as the force to accelerate is applied by us, just opposite force to retardate is applied by the friction.
Consider a highly smooth surface, in this case also book will come to rest when we remove the applied force but it will cover greater distance as compared to the previous case because now the surface is smoother. Consider that there is a floor which is so much smooth that it does not provide any friction force and the floor is circular. If we apply force, it will continue to move in this circular path indefinitely until some external opposite force is applied to stop it.
If the summation of all the applied forces to an object is zero, then the change of the velocity is zero.
If $\sum$ F = 0
Inertia of the object can be defined as the tendency of an object against any attempt or effort or force to change its velocity.
This basic law of Newton law is also called as the Law of Inertia.
Mass is the Quantitative Measure of Inertia
If you want to start a car by pushing it, you find that it takes a very large force to overcome its inertia. On the other hand, only a small force is needed to start a child's express wagon. The difference between the car and the express wagon is the difference in mass. The car has a large mass, whereas the wagon has a small one.
The inertia of an object provides a way of defining mass. The greater the mass of an object, the greater its tendency to keep at a constant velocity. Therefore, mass is often defined as the quantitative measure of the inertia of an object. Like the definition for force, this is an operational definition. It tells you that you can decide which of the two masses is greater by observing which one speeds up less when you apply the same force to each for the same length of time.
The first law of motion and inertia also applies to objects in motion. For example,
- When you pedal a bicycle, you accelerate to make the bicycle move forward. Because the forces acting on the bicycle are balanced, the bicycle and the rider keep moving at the same speed and in the same direction. The bicycle has inertia.
Inertia can be overcome only if the rider applies some type of force. The outside force could result from pushing the pedals, turning the handle bars, or using the brake.
You must have observed that when people alight from a moving train, they continue to run alongside the train. If they were to stop at once, the feet would come to rest suddenly but the upper part of the body would still be in motion and they would tend to fall forward.
When food packets are dropped from an aircraft, they do not hit the ground vertically below but fall on the ground further ahead. When the food packets leave the aircraft, they are moving at the speed of the aircraft. Due to inertia of motion they continue to move forward while falling. Hence they touch the ground further ahead.
It is a matter of common experience that objects at rest do not begin to move of their own accord. If we place an object in a certain place we expect it to remain there unless a force is applied to it. Some simple parlor tricks are based on this principle. For example, if a pile of droughts or pennies is placed on a table the bottom one can be removed without disturbing the remainder, simply by flicking it sharply with a piece of thin wood or metal. In this case the force of friction between the bottom disc and the one in contact with it, acts for too short a time to cause any appreciable movement of the discs above. Incidentally, the use of a ruler is not recommended for this experiment, as the blows it receives will not improve its straightedge.
(a) The bottle and coin trick is equally effective in demonstrating the same effect. A coin is put on a card and placed over the mouth of a bottle. When the card is flicked away with the finger the coin drops neatly into the bottle. For success in performing this trick the finger should move in horizontal plane so that the card is not tilted.
Place a coin on a smooth card over a water glass. Give the edge of the card a sharp blow with the flat side of a ruler. The card will be knocked off the glass, but the coin will fall into it.
(b) Pile up smooth blocks and give the bottom one a sharp rap with the edge of a ruler. The bottom block will sail out, and the rest of the stack will not be toppled but will drop down still in a pile.
A body moving with uniform velocity in a straight line tends to go on moving forever without coming to rest. The fact is that no one has yet found a mean of eliminating all the various outside forces, which can retard a moving body.
(c) A person riding a bicycle along a level road does not come to rest immediately as he stops pedaling. The bicycle continues to move forward, but eventually it comes to rest as a result of the retarding action of air resistance and friction. In a collision between two motor vehicles the passengers are frequently injured when they hit the windscreen. An external force stops the vehicle, but not the passengers who simply continue their straight line motion in accordance with Newton's first law.
(d) When a bullet is fired from a gun, its motion is opposed both by air resistance and the pull of the earth. Sooner or later it returns to the earth, but it would be reasonable to suppose, if air resistance and gravitation could be eliminated, the bullet would go on moving in a straight line forever.
(e) When a train suddenly starts, the passengers tend to fall backwards. This is because the lower part of the body which is in contact with the train begins to move while the upper part of the body tends to maintain its position of rest. As result, the upper part tends to fall backwards.
(f) Make a stack of 4 washers which are placed above the surface of the floor or book. We can say that the tower of washers is formed. Apply force on the bottom washer and note the results. Bottom washer will move forward when force is applied to it and come out from the stack but other three will remain as such and follow the law of inertia and will remain at the rest.
Everyday Applications of Newton's First Law
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Consider the case, if the gravitational force between the sun and other planets becomes zero, then what type of path would be followed by the planets.Result: All of the planets will start moving in straight line with zero acceleration or constant velocity.
Consider a spaceship which is traveling in the space. Propulsion system in the spacecraft is required to change its velocity from one value to other. Now, if the propulsion system is shut down and remains off, the spacecraft will continue to move at the constant velocity without any acceleration and far away from any planets or other matter.