Newton’s First Law of Motion

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Newton’s laws of motion are three physical laws that, together, laid the foundation for classical mechanics.  In this article, we shall discuss Newton’s first law of motion and the concept of inertia of a body.

Statement of Newton’s First Law of Motion:

Every material body continues to remain in its state of rest or state of uniform motion in a straight line unless acted upon by an external unbalanced force to change the state of motion. This law is also called as law of inertia.

Explanation: This law has two parts, the first part is giving us the concept of inertia while the second part helps us in defining the force. The first part indicates that if a body is at rest, then it cannot start moving by itself. Similarly, if the body is in state of uniform motion in a straight line cannot increase or decrease its velocity or can not change the direction by itself. Thus each body has the inability to change the state of rest or state of uniform motion along a straight line by itself this inherent property of body is called inertia of a body. The inertia of a body depends on the mass of the body. Thus mass is a measure of inertia of a body.

The second part of the law helps us in defining the force. In the first part, we have seen that a body can not change its state of motion on its own but some external physical quantity is required to do so. This external physical quantity which is required to change the state of motion of a body is called force.

Concept of Inertia of a Body:

• The tendency of the body to oppose the change of state of rest or state of uniform motion is called inertia of the body.
• If no unbalanced force acts on a body then the body at rest remains at rest. This inertia is sometimes referred as the inertia of rest.
• If no unbalanced force acts on a body then the body in uniform motion along a straight line remains in uniform motion along the same straight line. This inertia is sometimes referred as the inertia of motion.
• The tendency of a body to continue to move with uniform motion in a linear direction is called inertia of direction.

Examples of Inertia of Rest:

Example: A coin is placed on a smooth card which serves as a lid on a glass. When the card is pulled suddenly in the horizontal direction the coin falls into the glass. (Inertia of rest)

Explanation: When the card is pulled horizontally it acquires motion due to pulling force. But as no force is acting on the coin in a horizontal direction.  The coin initially at rest on the card due to inertia remains at rest. Thus gets separated from the card. Now there is no support at the bottom the card falls in the glass under the action of gravity.

More Examples of Inertia of Rest:

• A cyclist riding along a level road does not come to rest immediately after he stops pedaling.

• On striking the coin at the bottom of a pile of carom coins with a striker, this coin only moves away, while the rest of the pile remains at the original position.
• When a hanging carpet is beaten with a stick, the dust particles start coming out of it. When a carpet is beaten by stick the carpet is set into motion. But due to inertia, the dust particles remain at rest. Thus they get separated from the carpet.
• On shaking or giving jerks to the branches of a tree, the fruits fall down. When branches are shaken in one direction, the fruits and leaves due to inertia remain at the original position due to inertia of rest. This causes the breakup of the stalk and they fall down.
• When a bullet is fired at a glass window, a hole is formed in it. This is because only that part of the glass moves with the bullet, where the bullet hits the glass. The remaining part due to the inertia remains in its position. Thus bullet is able to form a hole in the glass window due to the inertia of window pane.
• A magician snatches a table cloth from under a full set of tableware. When the table cloth is pulled it is set into motion, but the tableware due to inertia of rest remains on the table.
• When the local train starts or stops suddenly, sliding doors of some compartments may open or close.

Example of Inertia of Motion:

Example: When a stationary bus starts moving passengers in the bus get reclined back similarly when bus moving with uniform velocity stops suddenly passengers move forward. (Inertia of motion)

Explanation: When the bus is stationary the passengers are also stationary. When the bus starts moving the part of the body (lower part) in contact with bus starts moving, but due to inertia the upper part remains stationary and thus he gets reclined back. If he is standing he will fall backward. When the bus is moving with uniform motion in a straight line the passengers have the same motion. When the bus stops, the part of the body (lower part) in contact with bus stops, but due to inertia the upper part continues to move forward and thus he moves forward. If he is standing he will fall forward.

More Examples:

• When a passenger jumps out of a moving train he falls down. This is because as soon as the person leaves the running train, his velocity is the same as that of the train. When his feet come in contact with the ground, the lower part of his body is brought to rest, but the upper part of the body continues to travel with original velocity. This makes him fall in forward direction. To avoid this he has to run in the forward direction till his velocity is reduced to zero.
• A ball thrown vertically upward by a person in a moving train comes back to his hand. The reason is that the moment the ball was thrown, the ball was in motion along with the person and the train due to the inertia of motion. So during the time the ball remains in the air, both the person and the ball move ahead by the same distance. This makes the ball come back to his hand on its return.
• Athletes run before taking a long jump in order to increase his speed, and thereby his inertia of motion. The increased inertia of motion enables him to jump a longer distance.
• Athletes (Long jumpers / Javelin throwers /Shot putters)  often fail to stop themselves before fault line because of the inertia of motion the upper part of the athlete’s body continues to move in the forward direction while the lower part comes to halt. Thus he may not be able to stop at the fault line and crosses it.

Example of Inertia of Direction :

Example: When a vehicle takes a sudden turn towards the left, the person seated inside the vehicle is pushed towards the right. (Inertia of direction)

Explanation: When a vehicle takes a sharp left turn it changes the direction. Whereas the person seated inside tends to move in the original direction due to inertia. Thus he is pushed towards the right.

More Examples:

• When a bus makes a turn around a corner, the passengers have to hold on to some support to prevent themselves from swaying. The bus and passenger both are in a state of motion. When the bus changes the direction, the passengers continue to move in the same direction due to the inertia of direction. If passengers do not hold to some support would get thrown in that direction.

Notes:

• If a body is at rest, then the net force acting on the body is zero.
• If a body is moving in uniform motion in a straight line, then the net force acting on the body is zero.
• If a body is neither at rest nor in uniform motion, then the net force acting on the body is not zero.
• If a body is changing direction, then the net force acting on the body is not zero.
• If the net force acting on a body is zero, then the body must be at rest or uniform motion in a straight line.
• If the net force acting on a body is not zero, then the body is neither at rest nor in uniform motion in a straight line.

Stopping of a Moving Vehicle:

If the car engine is switched off or brakes are applied to stop a car, the car does not stop at once. Sometimes a driver has to apply emergency brakes. The time interval between seeing the obstacle and actually applying break is called time of response or time of thinking. The distance traveled by car during this period is called the distance of thinking. The time interval between applying brakes and the actual stopping of the car is called the time of braking. The distance traveled by car during this period is called braking distance. The sum of thinking distance and braking distance is called the stopping distance. Thus to avoid an accident the stopping distance should be less than the distance of the obstacle from the point of seeing it.

Winnowing of Grains:

By winnowing the grain get separated from the husk. Winnowing is an agricultural process in which the grain and husk are separated from each other. Grain has a larger mass than the husk. Thus the inertia of grain is more than that of husk. Thus more force is required to change their path of motion. When dropped from height in a gentle wind, due to higher inertia they just fall down vertically. The husk particles have negligible mass and very negligible inertia. Thus small force is required to change their path of motion. When dropped from height in a gentle wind, they get carried away in the direction of wind through some distance. Thus the husk and grain get separated.

Luggage on the Roof of Bus is Tied:

It is advised to tie the luggage with ropes on the roof of the bus. When a bus is moving (especially with a high speed) on the road suddenly stops or suddenly changes its direction, the luggage on the top due to inertia of motion and direction continues to remain in the motion or in the same direction of motion. As a result, the luggage may get thrown out from the bus roof if it is not tied with the rope.

Drying of Cloths By Shaking:

When a wet cloth is shaken, the water particles start coming out of it. When the cloth is shaken the cloth is set into motion. But due to inertia, the water particles remain at rest. Thus they get separated from the wet cloth. Thus cloths can be dried early.

A person sitting in a car tries to move the car by applying force to its walls. Will the car move?

Newton’s first law states that “Every material body continues to remain in its state of rest or state of uniform motion in a straight line unless acted upon by an external unbalanced force to change the state of motion.” Thus two move a body at rest some external unbalanced force is required. In this case, the force applied by the person is internal. Hence the car will not move.

Examples where low inertia is preferred and an example where high inertia is preferred (recommended):

Carpenter works with wood and nails. To drive nails in wood, less force is required. Thus low inertia of hammer is recommended. Thus carpenter’s hammer is an example of low inertia. A blacksmith works with iron, steel. To change the shape of iron or steel, large force is required. Thus high inertia of hammer is recommended. Thus blacksmith’s hammer is an example of high inertia.

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