In this video i want to clarify two ideas

that we talk about on a regular basis

but they are really kind of muddled up in our popular language

And these are the ideas of mass and weight.- Mass and weight

and first I `ll tell you what they are and then we'll talk about how they are muddled up

So mass is literally, there's a couple of ways to view mass.

One way to view mass - this is not a technical definition

but will give you sense of it, how much stuff there is

so this is similiar to saying "matter". So if I have more molecules

of a given mass; I will have a total of more mass.

So if I have more atoms, I will have more mass.

How much stuff there is of something,

And I want to be careful with this definition right here because there are other things that aren´t, what would be what we traditionally associate with matter

and I don't want to start going into more fancy physics

that still will exhibit mass, so another way to define mass is

how does something react to a specific force,

we have already learnt it in the second law.

if you have a given force, and you have more mass

you'll accelerate less and if you have less mass,

you'll accelerate more.

So, how something responds to force?

how something responds to force, to a given force?

Something with lower mass will acelerate more for a given force

and something with higher mass will accelerate less

Now weight is the force of gravity on a mass or on an object. So this is the force of gravity on an object,

Just let`s think about, the differences right here,

Let`s think about I guess myself sitting on Earth,

so if I am on Earth, my mass is 70 kilograms, my mass (let me use a new colour), my mass is 70 kilograms

There is 70 kilograms of stuff that constitute Sal.

But my weight is not 70 kilograms

you often tell, here people say " I weigh 70 kilograms",

and that's all right in just conversational usage but that is not technically correct.

Because weight is the force that Earth is pulling down, or I should say

the force of gravity on my mass,

and so my weight, the weight is going to be equal to the gravitational

field at Earth , let me just....

We did hopefully watch the video on gravity, if you haven't, feel free to watch it,

the gravitational attraction between two objectss, so the force of gravity between two objects is going to be a big G.

the universal gravitational constant

times the mass of the first object times the mass, (let me actually do it), times the mass of the first object

times the mass of the second object , divided by the distance that separates

the two objects squared

And if you are on Earth,

if you take all of the stuff over here combined,

so if you say that this right here is the mass of Earth,

if you say this right here is the distance from the centre to surface of Earth

`cause there is where I'm sitting right now.

distance from centre to surface of Earth

Then all of this stuff over here simplifies to what

is sometimes called lower case g,

and lower case g is,

and I'm just rounding it here,

is 9.8 meters per second squared.

So the force of gravity for something near the surface of the Earth,

is going to be this quantity right over

here times the mass, times its mass.

So my weight on the surface of the Earth is this 9.8 meters per second squared

times my mass, times 70 kilograms

so this is going to be,

And I could get my calculator, why don't I just get my calculator rather than do the maths? I was going to round to 700 but let 's just actually calculate it,

so we have 9.8 times 70 kilograms,

so we have 686. so this gives me 686 and then the units

are kilograms meters per second squared, and these units kilograms meters per second squared are the same thing as a NEWTON, a NEWTON

So my weight, and you will never hear people say this,

my weight on the surface of the Earth is 686 Newtons, 686 Newtons

and notice i just said that is my weight on the surface of the Earth,

because as you can imagine weight is the force due to gravity

on an object, on on a mass.

So if you go some place else, if I go to the moon for example

my weight will change, but my mass will not.

This is the weight on Earth.

If I were to take my weight on the moon, the weight on the moon

And I've had to look this up before the video, and you can verifiy this for me if you like,

But I have been told that the gravitational force on the moon,

or the gravitational attraction at the surface on the moon

is about 1/6 of that of the surface of the Earth,

so my weight on the moon will be roughly

1/6 of my weight on the Earth.

So times 686 Newtons, so that gives me a little bit over, what is that? 114? , maybe

I'll just get the calculator out.

my brain operates a little bit slower,

while I'm recording videos , I got that right.

Yes 114 approximately, 114 Newtons.

This is the thing I really want to emphasize,

weight is the force due to gravity on a object

your weight changes from planet to planet you go on.

Your weight would actually even change as you, as you, if you went to a very high altitude,

because you are getting slightly farther, you would be inmesurably small.

You would be slightly farther from the centre of the Earth

your weight would change an imperceptible amount,

in fact because the Earth is not a perfect sphere,because the Earth is not a perfect sphere, it's slightly, it's often referred to

an oblix spheroid.

Your weight is actually slightly different, your weight is actually slightly different on different parts of the Earth.

If you went to the Poles versus the Equator

you would have a slightly different,a slightly different weight.

Your mass does not change, it doesn`t matter where you go,

assuming you don`t have some type of nuclear reaction going on inside of you.

Your mass does not change, your mass does not change depending on where you are.

Now you might be saying hey, look! You know, I don't deal with kilograms and Newtons

and all of this, I operate in America and in America we talk about pounds

Is pounds aproppiate? And yes, pounds is a unit of weight

so if I say, if I say that I weigh one hundred and sixty pounds,

this is indeed weight

I'm saying that the force of gravity on me

I'm saying that the force of gravity on me

is a hundred and sixty pounds but then you may say what is mass then?

if you are talking about the English system or sometimes called the Imperial System

and here I'll introduce you to a concept that very few people note

kind of a good trivial concept, the unit of the mass, the unit of mass

Let's just speak clear in the Imperial System

mass is called the slug, mass, the unit of mass is called the slug

mass the unit of mass is called slug so if you are so many slug

so if you wanted to figure out how many slugs you are, so your weight, the force of gravity on you is one hundred and sixty pounds,

one hundred and sixty pounds, this is going to be equal to

if you are to take, if you are to calculate, if you are to calculate all of the stuf,f the force of gravity

on the surface on the Earth but if you are doing it,if you are doing it in Imperial units

instead of getting 9.8 meters per second squared

instead of getting 9.8 meters per second squared, you would get 32 feet per second squared

you would get 32 feet per second square which

also the acceleration on, near the surface of the Earth

due to gravity in feet and seconds it's supposed

to meters and seconds and this is times your mass,

times your mass in slugs, in slugs,

so to figure it out you divide both sides by thirty

two per second squared so let's do that,

let's divide both sides by thirty two feet per second squared, let's divide both sides by thirty two feet per second squared

cancels out and then let me get my calculator out so I have

one hundred and sixty pounds divided by thirty two feet per second squared

and I get exactly five, (I should be able to that in my head), so I get five, and the units here,

the numerator I have pounds

I have pounds, I have pounds and then I'm dividing

by feet per second square that's the same thing as multiplying by

second squared per feet and these units five pounds second squared

over feet this is the same thing as a slug, so if I were one hundred

and sixty pounds my mass is going to be equal to, my mass is going to be equal to

five slugs, so if my mass

is seventy kilograms, mass is seventy kilograms, my weight

Explained well,and understandable with practice working through calculations.