Tutorial - Newton's 1st law

Newton's First Law and Inertial Frame of Reference
(Prerequisites:- frame of reference, speed, velocity, acceleration)

Newton's first law says: -
"Seen from an inertial from of reference, a particle will continue to remain in its state of rest or of uniform motion so long total force on it is zero."
To put another way: -
"There exists a frame of reference, called inertial frame of reference, seen from which a particle will continue to remain in its state of rest or of uniform motion so long total force on it is zero."

Uniform motion means motion in a straight line with constant speed.
Often books do not mention "inertial frame" when quoting Newton's first law but "inertial frame" is implied.

The tendency of a particle to retain its state of rest or of uniform motion in the absence of zero total force is called as inertia. Therefore, Newton's first law is also called as the law of inertia.

Take some situations where Newton's first law is applicable: -
[*]You are in a bus, which is at rest. Suddenly the bus starts. You fall back.
[*]You are in a moving bus. Suddenly the bus stops. You fall ahead.
[*]You are traveling in a fast moving bus. You jump off the bus. You fall ahead and hurt yourself.

The above can be explained using Newton's first law. When the bus starts moving, then your body tries to maintain its state of rest. That is why you fall back.

When the bus stops, then your body tries to maintain its rest of motion. That is why you fall ahead.

If you jump from a moving bus, then just after jumping, your body tries to continue moving. That is why you fall ahead.

But, Newton's first law is not applicable in all frames of reference. Take an example where it is not applicable: -
You are in a stationary bus. You are looking at a tree. You find that the tree is also at rest. Now the bus starts moving. You see that the tree is moving in backward direction. You may say that the tree is not moving but the bus is moving. Fine, but in bus's reference frame the tree is moving. When the bus was at rest, then the tree was at rest. When the bus starts moving, then in the bus's frame of reference the tree starts moving.

There is no force on the tree which made it move. In the absence of a force, the tree should not have moved according to Newton's first law. Still it moved. This means that Newton's first law is not applicable here. Why is it not applicable? It is not applicable because there is a force on the frame of reference i.e. the bus. The bus moves because the ground exerts force on it to make it move.

Newton's first law is not applicable in any frame on which there is some force. Such a frame is called non-inertial frame. So, a frame on which total force is not zero is called non-inertial frame. A frame on which total force is zero is called inertial frame. Newton's first law is applicable only in inertial frames of reference.

If you have found one inertial frame, then you can easily check if another frame is inertial or not. This is because a frame which is at rest or moves at constant velocity with respect to an inertial frame is also inertial. A frame which accelerates with respect to an inertial frame is a non-inertial frame.

If you know from experiments that Newton's first law is applicable in a frame, then you know that it is an inertial frame. Therefore, Newton's first law can also be used to define inertial frame as follows: -
An inertial frame is the one in which Newton's first law is applicable.
You have found three ways of identifying an inertial frame: -
1. An inertial frame is the one on which total force is zero.
2. An inertial frame is the one in which Newton's first law is applicable.
3. Any frame at rest or moving at constant velocity with respect to an inertial frame is also an inertial frame.

Please go through the above and give me your feedback.

its really wonderful👏. There is way of explaining which different People have different😊. but the way you explained is really awesome👍🏼.

May i explain newton 2nd law😊. I am not genius like u 😛 but i would like to give it a try😊 and need a genius like urs😛 feedback😊😊

Everyday Applications of Newton's First Law:-

There are many applications of Newton's first law of motion. Consider some of your experiences in an automobile. Have you ever observed the behavior of coffee in a coffee cup filled to the rim while starting a car from rest or while bringing a car to rest from a state of motion? Coffee tends to "keep on doing what it is doing." When you accelerate a car from rest, the road provides an unbalanced force on the spinning wheels to push the car forward; yet the coffee (which was at rest) wants to stay at rest. While the car accelerates forward, the coffee remains in the same position; subsequently, the car accelerates out from under the coffee and the coffee spills in your lap. On the other hand, when braking from a state of motion the coffee continues forward with the same speed and in the same direction, ultimately hitting the windshield or the dash. Coffee in motion tends to stay in motion.

Have you ever experienced inertia (resisting changes in your state of motion) in an automobile while it is braking to a stop? The force of the road on the locked wheels provides the unbalanced force to change the car's state of motion, yet there is no unbalanced force to change your own state of motion. Thus, you continue in motion, sliding along the seat in forward motion. A person in motion tends to stay in motion with the same speed and in the same direction ... unles acted upon bu an unbalanced force, of a seat belt. Yes! Seat belts are used to provide safety for passengers whose motion is governed by Newton's laws. The seat belt provides the unbalanced force which brings you from a state of motion to a state of rest. Perhaps you could speculate what would occur when no seat belt is used.

There are many more applications of Newton's first law of motion. Several applications are listed below. Perhaps you could think about the law of inertia and provide explanations for each application.

1Blood rushes from your head to your feet while quickly stopping when riding on a descending elevator.

2The head of a hammer can be tightened onto the wooden handle by banging the bottom of the handle against a hard surface.

3 A brick is painlessly broken over the hand of a physics teacher by slamming it with a hammer. (CAUTION: do not attempt this at home!)

4 To dislodge ketchup from the bottom of a ketchup bottle, it is often turned upside down and thrusted downward at high speeds and then abruptly halted.

5 Headrests are placed in cars to prevent whiplash injuries during rear-end collisions.

6 While riding a skateboard (or wagon or bicycle), you fly forward off the board when hitting a curb or rock or other object which abruptly halts the motion of the skateboard.

Good, Rajnish. All are perfect. But I think your first post should be reorganised to make it easier to understand by using small sentences. Otherwise, everything is perfect.

Great tutorial - definitely something very easy to understand. 😃