How does momentum differ from inertia?
First, lets look at the equation for momentum:
This equation shows that momentum is directly proportional to an object's mass and velocity.
Everything has mass; and thus, everything that moves must have a momentum because its mass in motion!
Now if momentum is "mass in motion", then what's inertia ?
Inertia is the RESISTANCE an object has quality of an object to keep that object at rest if it is at rest and to keep it moving in uniform motion if it is moving in uniform motion.
Now what does that even mean?!?!
For example, if you're driving down a road at a really high speed you'll feel as though you are being "pushed" back into your seat as the car moves forward. Likewise, when you brake hard, your body lurches forward. Also, turning right your body leans to the left. and turning left your body leans to the right. That is inertia !
The more mass an object has, the more resistance it has, the more inertia it has.
Lets say you have two bricks. You wish to determine which one is the heavier one just by pushing them. You push brick 1 with a little bit of force until it starts moving. You then push brick 2 with more force than the first one to get it moving. Brick 2 had the most resistance to being moved and thus breaking its state of rest = more inertia = must be more mass = brick 2 is the heaver one.