Concept of Force and Inertia Physics Notes

Concept of Force and Inertia:
Force: It is a push or pull that either changes or tends to change the state of rest or of uniform motion of a body or the direction of motion of a body.

It is our common observation that an object such as a chair lying in a room or a vehicle parked outside the house remains at rest unless a push or a pull is given to it. Such an object cannot move at its own. In other words force has to be applied in order to move an object at rest.

Also if an object is moving along a straight line with some velocity, it is found that force is required to change its direction of motion or the magnitude of its velocity. In other words force is an agent which causes acceleration. However, in certain cases the acceleration caused by the force may be practically zero. For example, if we push a stationary truck, it may not move. In such cases we say that the force merely tends to cause acceleration zero.

Hence, force is a push or pull which produces or tends to produce a motion in a body at rest, stops or tends to stop a body which is in motion, increase or decreases the magnitude of the velocity of the moving body or changes the direction of motion of the moving body.

To describe the applied force on a body, the following facts are need to be determined.

The magnitude of applied force.
Direction of applied force.
Point of position of applied force.

Inertia: In the absence of external force the inability of a body to change its state by itself is called Inertia.
It is the property of an object which resists the change in the state of linear motion. It is equal to the mass of the object.

As said above an object such as a chair lying in the room remains at rest unless an external force is applied on it. It can be set into motion only by applying force on it. In other words, if we do not apply a force on the stationary chairs, it will not move at all. This inherent property of the objects that they do not change their state, unless acted upon by an external force is called Inertia.

It was the general belief centuries before; that force is necessary even to keep a body in motion with uniform velocity. With his simple experiments with inclined planes. Galileo proved that no force was required to keep a body moving with uniform velocity provided friction is not present. Therefore, it is the inherent property of the object that they remain moving with a constant speed along a straight line unless acted upon by an external force. Thus, it is due to inertia that a body cannot change its state of rest or a uniform motion,by itself. It is sometimes also called the Galileo Law of Inertia.

Kind bf Inertia:

The inertia of rest: It is defined as the inability of a body to change by itself its state of rest. This means a body at rest remains at rest and can not start moving by its own. This is called inertia of rest.
The inertia of motion: It is defined as the inability of a body to change itself its state of uniform motion. This means that body in uniform motion can neither accelerate nor retard by its own. This is called inertia of motion.
The inertia of direction: It is defined as the inability of a body to change by itself the direction of motion.
Force produces acceleration and retardation in moving body.
Inertia is not a physical quantity, it is only the inherent property of a body which depends upon the mass of the body.
Inertia has no unit and dimension.
Two bodies of same mass have equal inertia because the inertia only depends on the mass. It does not depend upon the velocity and size of the body.
Inertia means resistance to the change in its state.