The concepts of momentum and inertia are often confused possibly due to the similarity of their definitions. Inertia is generally described as an object’s resistance to motion, while momentum is the tendency of an object to continue moving.
Both have implications for linear motion applications, but while inertia is a fundamental sizing parameter, momentum isn’t directly addressed in system calculations.
To distinguish between inertia and momentum and find out why that is, we’ll look at the definitions and uses of each.
Inertia vs Momentum
The terms inertia and momentum can both be used to describe how difficult it is to change the state of motion of an object. The main difference between inertia and momentum is that inertia does not depend on the object’s velocity, whereas momentum depends on the object’s velocity.
What is Inertia?
Inertia is a term that describes the resistance of an object to change its velocity. This includes the resistance shown by the objects at rest to start moving, and the resistance by a body in motion to change its speed and/or its direction of motion.
Inertia is usually characterized by a body’s mass. It is more difficult to change the state of motion of more massive objects, so they have more inertia.
When a bus suddenly applies brakes, the passengers may get “thrown” forward. Here, the bus has not exerted a forward force on the passengers! Instead, this is an effect of inertia. If passengers are not making enough contact with the bus, a sufficient force cannot be applied on the passengers to change their state of (forward) motion, and so they will try to keep moving at the same speed in a straight line.
What is Momentum?
Momentum is simply defined for an object as the product of the object’s mass and velocity. Since the mass of an object is an indication of the object’s inertia, depends on an object’s inertia. The resultant force on an object can be given by the rate of change of momentum of the object.
In this sense, an intuitive interpretation for momentum could be, “the amount of force required to bring a moving object to rest in one second“.
Note that momentum depends on the state of motion, whereas inertia does not. An object at rest will have inertia, whereas it will not have momentum.