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 ♥♥♥♥ing 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).

Now since they have the same mass they will have the same weight, since gravity is essentially is a constant at any given altitude on Earth, the water and the air will have the same weight because they have the same mass and because gravity is the same. Therefore they would press down on the Earth with the same force as well. So lets go with a scenario of you are holding the objects. To make this example easier to understand lets do this. Lets say we have two boxes that are the same size with no mass, they also don't bend or stretch. Now lets say in one you put in the kilogram of water into it and in the other box you put the 1 kilogram of air into it. Now there is 1 kilogram of stuff air in once box and 1 kilogram of water in another. So now you go to pick up the boxes, they both have 1 kg (kilogram) of mass inside them. That means the will weigh (force caused by gravity) of about 9.8 Newtons which is about 2.2 pounds for those who use imperial units.

Now just ♥♥♥♥ing substitute steel/lead for water and feather for air and you get the same ♥♥♥♥ing result.

When idiots talk about which is heavier they talk about which would hurt more if it fells on you. A block or ball of ♥♥♥♥ing metal versus a clump of ♥♥♥♥ing feather that not held together in any way and thus would not act as one unit/mass and then turn into a bunch of ♥♥♥♥ing individual feathers than do not weight 1 pound individually. Now, because of the shape and density of the ♥♥♥♥ing feather there is a ♥♥♥♥ ton of drag force (air resistance) acting on those individual feather prevent them from accelerating as fast. If you were to hold them they would exert the same amount of force/weight on hands. If they were to drop on your head they would exert the same force because they have the same mass and gravity is the same everywhere on Earth (for practical purposes). The difference being that if steel/lead fall on your head, it would be a smaller object because it is denser and because it is a single object held incredibly tightly together due to inter-atomic/molecular forces. Compare that to a bundle of feathers not held together, all of a sudden you drop this bundle then all of a sudden they split up because they are a bunch of ♥♥♥♥ing individual feather that are not held together. They will end up separating then all of a sudden you are hit by a bunch of individual feathers that do not weigh a pound that are hitting you, they are not even accelerating at normal gravitational acceleration because of the drag force (air resistance) acting on. Of course the pound of steel is going to hit ♥♥♥♥ing harder, its one object that is not being affected by drag (air resistance) to the same degree as the bunch of individual feathers not being held together (much, much less than the feathers). Even if the feathers were held together it still wouldn't hit harder because it is less dense and thus needs more volume to get to 1 pound, because it takes up more space there is more area for friction/drag (air resistance) to act upon thus it won't accelerate as fast in free fall. Just because one would hit hard in that situation does not mean one weighs more than the other because like I said before it is a product of mass and gravity and because gravity is constant and in this situation they have the same as well they will weigh the same. 1 kilogram of steel multiplied by the acceleration due to gravity gives you 9.8 Newtons (2.2 pounds). 1 kilogram of feathers multiplied by the acceleration due to gravity gives you 9.8 Newtons (2.2 pounds). Therefore, a pound (or kilogram in my example) of feather WEIGHS AND HAS THE SAME MASS as a pound (kilogram in my examples) of steel/lead.

TL;DR - If you don't understand that pound of feathers and a pound of steel weigh the same there is no TLDR, you need to read this whole thing. I could make this longer but if you don't understand the basic concept of mass and weight then me making this longer will not do you any good.

Anyone who does not understand this is a ♥♥♥♥ing idiot and needs to go back to middle school or high school where you should have learned this ♥♥♥♥, if you school never taught you this then everyone in charge of the decision making when it comes to curriculum should be replaced with someone who isn't ♥♥♥♥ing stupid.