The tendency of a body to remain at rest or if it is moving to continue moving is called inertia whereas momentum is the product of mass and velocity.

Inertia:
Inertia refers to the property of an object to resist any change in its state of motion, whether it is at rest or in motion. It is a fundamental concept in physics and is closely related to the mass of an object. Inertia can be thought of as a measure of how difficult it is to change an object's velocity. The more massive an object is, the greater its inertia and the more force is required to accelerate or decelerate it.

Momentum:
Momentum, on the other hand, is a vector quantity that describes the motion of an object. It is defined as the product of an object's mass and its velocity. Momentum depends on both the mass and the velocity of an object, which means that two objects with the same mass can have different momenta if they are moving at different velocities.

Differences:
There are several key differences between inertia and momentum:

Definition:
- Inertia is the property of an object to resist changes in its state of motion.
- Momentum is a measure of an object's motion and is calculated as the product of its mass and velocity.

Quantities:
- Inertia is a scalar quantity and is directly related to an object's mass.
- Momentum is a vector quantity that includes both magnitude and direction.

Change in Motion:
- Inertia describes an object's resistance to changes in its state of motion.
- Momentum describes an object's motion and how difficult it is to change that motion.

Relation to Mass:
- Inertia is directly proportional to an object's mass. The greater the mass, the greater the inertia.
- Momentum is also directly proportional to an object's mass. Increasing the mass increases the momentum.

Relation to Velocity:
- Inertia does not depend on an object's velocity. It only depends on its mass.
- Momentum depends on both an object's mass and its velocity. Increasing the velocity increases the momentum.

Implications:
- Inertia is responsible for objects staying at rest or continuing to move in a straight line at a constant velocity unless acted upon by an external force.
- Momentum is involved in collisions and interactions between objects. It determines how objects will move and react to forces.

In summary, inertia is a property of an object that resists changes in its motion, while momentum describes the motion of an object. Inertia depends only on an object's mass, while momentum depends on both mass and velocity.