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So many people seem to think a pound of feathers and a pound of steel weigh the same but this is not true and is the kind of ♥♥♥♥ you should have learned in primary and secondary schools.

Lets start with the basics.

Mass: It also tells you how much "stuff" is in it. Now by stuff I kind of mean atoms and molecules but different atoms and molecules do not weigh the same, so the amount of "stuff" is dependent on the thing we are talking about. It is also a measurement of inertia, which is an object resistance to acceleration (or deceleration). Units to measure mass include but are not limited grams, pounds, tons, kilograms, stones, etc.

Volume: How much room something takes up (3-d space) units include (cubic centimeters, cubic meters, liters, etc)

Density: Mass of an object per given unit of volume this can be found by dividing mass by the volume it occupies. (g/L which is a gram per liter, could also be gram per cubic centimeter,)

Gravity: Caused by mass, all mass exerts a gravitational pull. the more mass the more "gravity" it will have. Gravity is a constant pretty much every where on earth, it does decrease as you increase in altitude by it is negligible, even if you were to stand on top of mount Everest there isn't too much of a difference with the acceleration due to gravity being 9.77 m/s/s, so its negligible unless you are talking about extreme masses, but this difference is not important to the subject at hand

Velocity: Your displacement per unit of time(i.e. seconds)

Speed: Distance per unit of time (i.e. seconds)

Force: I am not talking about Star Wars force, I am talking about physics force. A vector, the product of mass and acceleration, and I mean it literally, multiply mass and acceleration and you get force. Now, gravity causes an acceleration so your "weight" is technically a force caused by the gravity of Earth which in free fall accelerates you by 9.8 m/s/s (9.8 meters per second per second), meaning you speed up by 9.8 meters per second every second. The unit for this is Newtons in Metric and pounds, the imperial units are ♥♥♥♥♥♥♥ stupid when it comes to force cause there is stones, pounds (which is also what they use for mass), and slugs and some other ♥♥♥♥, I am not going to use them)

Momentum: A product of mass and velocity, found by multiplying mass and velocity.

Now lets get into this, a kilogram of one this is going to have the exact same mass as a kilogram of another thing. Lets look at water and "air" for example. Mass is a measurement of how much stuff is in something and density tells you how much stuff is in something per unit of volume (so you could say how packed in the stuff is per unit of volume). Water is a lot more dense (meaning there is more stuff per given unit of volume, or it is more "concentrated") than air, so you will need less water in terms of volume to get a kilogram than you would air (we are talking about air at at sea level pressure).