MT,
Inertia: (First Law of Motion_given by Newton) natural property of a material body due to which they oppose any change in their original state.
For e.g. as a car brakes, we jerk forward. This is because our body was already in forward motion, and wants to retain it. However, we are forced to stop as our carriage has stopped. But, our rebellious inertia property wants to continue so we are simply pushed forward instead!
Momentum:(it is the product of mass and velocity)it is a vector quantity, dirtection same as that of the velocity. In layman language, momentum is the amount of impact produced by something onto sumthing. Umn.. you get it? Although the SI units are Ns, it can also be represented as kg*m/s
Kinetic Energy:
[Umn, you want to grasp the concept, right? So, before I explain any of that, get this. In any isolated system (hypthetically) the Total Energy always remains the same; but can be converted from one form to another.Energy and Work are SCALAR quantities, both with the same units. However the Physical signifcance is diffi. Energy is requred to do work. Moving an object causes that object to gain/lose momentum. And if an object is moved, work is said to be done. I hope you aren't muddled up by the bombardment of so many brief concepts.]
To expalin it simply, KE is basically the energy possesed/required by an object in horizontal motion. Be it vibrating molecules or a ball thrown by a player!
Hope I've Helped

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Pin~Jinx