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- 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
- six hundred and eighty six Newtons, so hopefully that clarifies things a little bit.

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