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First Law of Motion

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First Law of Motion - Lesson Summary




An object does not change its state of rest or  uniform motion by its own. The inability of any object to change its state is called inertia. Newton’s first law of motion gives the concept of inertia and force. According to the law, any object in the universe remains in its state of rest or uniform motion. Inertia is of three types, namely Inertia of rest, Inertia of motion and Inertia of direction. Mass of an object is an intrinsic property of matter.

Mass is a measure of inertia of an object. If an object does not change its position with respect to time and the surroundings, it is said to be at rest and else it is said to be in motion. The rate of change in displacement of an object is called velocity. If an object covers equal displacements in equal intervals of time, the motion is called uniform motion, else it is non-uniform motion.

Mass and Inertia
According to Newton’s first law of motion, inertia is the natural tendency of an object to resist any change in its natural state of motion.Inertia of an object is not a physical quantity, and hence, we can’t measure it directly. Therefore, it does not have any unit.

In other words, all objects resist a change in their state of rest or motion.

In a qualitative way, the tendency of undisturbed objects to stay at rest or to keep moving with the same velocity is called inertia. This is why, the first law of motion is also known as the law of inertia. Inertia is of the following three types.
 
Inertia of Motion
The natural tendency of an object to resist a change in its state of motion is called inertia of motion.

Example
We tend to remain at rest with respect to the seat until the driver applies a braking force to stop the motorcar due to the inertia of motion.
 
Inertia of Rest
The natural tendency of an object to resist a change in its state of rest is called inertia of rest.
 
Example
The dust particles fly away from the blanket when we jerk it due to the inertia of rest.
 
Inertia of direction
The natural tendency of an object to resist a change in its direction of motion is called inertia of direction.

Example
We tend to move out ward with respect to the seat at the cured roads as the vehicle takes a sudden due to the inertia of direction.

The mass of a body is the measure of its inertia. It means that heavier and more massive bodies offer more inertia than lighter and less massive bodies.

If a large body is at rest, then a large force is required to put it in motion. For example, even a small child can push a toy car. However, An adult also can’t push a loaded vehicle forward.
This is the reason why kicking a football is a pleasure, while kicking a large stone is very painful.




Galileo experimentally proved that objects in motion move with constant speed when there is no force acting on it. He performed many experiments with inclined planes. He noted that when a sphere is rolling down an inclined plane, its speed increases because of the gravitational pull acting on it. The speed of the sphere decreases as it climbs an inclined plane.

When two inclined planes are placed side by side, and the sphere that rolls down the first inclined plane is made to climb the second inclined plane, it comes to rest after reaching a certain height.

According to Galileo, if the force acting on the sphere is only gravitational force, then the height reached by the sphere must be equal to the height from which it was rolled. When the inclinations of the two planes are the same, the distance travelled by the sphere while rolling down is equal to the distance travelled by it while climbing up.

Now, if the inclination of the second plane is decreased slowly, then the sphere needs to travel over longer distances to reach the same height. If the second plane is made horizontal, then the sphere must travel forever trying to reach the required height.

This is the case when there is no unbalanced force acting on it. However, in reality, frictional forces bring the sphere to rest after it travels over a finite distance.

After further study, Newton, in his first law of motion, stated that all objects resist a change in their natural state of motion.

This tendency of resisting any change in the natural state of motion is called “inertia”.

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