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# Can someone explain the use of SI units and their prefixes - physics Watch

1. The spec for this section includes:
Fundamental (base) units.
Use of mass, length, time, amount of substance, temperature, electric current and their associated SI units.
SI units derived.
Knowledge and use of the SI prefixes, values and standard form.
The fundamental unit of light intensity, the candela, is excluded.
Students are not expected to recall definitions of the fundamental quantities.
￼￼￼￼￼Content
￼￼Dimensional analysis is not required. Students should be able to use the prefixes:
T, G, M, k, c, m, μ, n, p, f,
Students should be able to convert between different units of
the same quantity, eg J and ￼ , J and ￼ .

Can someone just explain SI units in simpleton terms.
2. BUMP!
3. (Original post by CorpusLuteum)
The spec for this section includes:
Fundamental (base) units.
Use of mass, length, time, amount of substance, temperature, electric current and their associated SI units.
SI units derived.
Knowledge and use of the SI prefixes, values and standard form.
The fundamental unit of light intensity, the candela, is excluded.
Students are not expected to recall definitions of the fundamental quantities.
￼￼￼￼￼Content
￼￼Dimensional analysis is not required. Students should be able to use the prefixes:
T, G, M, k, c, m, μ, n, p, f,
Students should be able to convert between different units of
the same quantity, eg J and ￼ , J and ￼ .

Can someone just explain SI units in simpleton terms.
it's just a measuring system based on kg (mass), m (length) and s (time)

the prefix notation is to spare people writing out numbers with a lot of zeros before or after the decimal point long handed.

e.g. 1000000 m = 1000 km = 1Mm

https://en.wikipedia.org/wiki/Metric...of_SI_prefixes
4. (Original post by Joinedup)
it's just a measuring system based on kg (mass), m (length) and s (time)

the prefix notation is to spare people writing out numbers with a lot of zeros before or after the decimal point long handed.

e.g. 1000000 m = 1000 km = 1Mm

https://en.wikipedia.org/wiki/Metric...of_SI_prefixes
more so to do with the fact that there were a lot of older measuring systems and that S.I units just make it standard across the board.
before metric measurements.
5. (Original post by sketchymofo2)
more so to do with the fact that there were a lot of older measuring systems and that S.I units just make it standard across the board.
before metric measurements.
(Original post by Joinedup)
it's just a measuring system based on kg (mass), m (length) and s (time)

the prefix notation is to spare people writing out numbers with a lot of zeros before or after the decimal point long handed.

e.g. 1000000 m = 1000 km = 1Mm

https://en.wikipedia.org/wiki/Metric...of_SI_prefixes
Ah, danke.

Do you guys know what this means:

'Fundamental (base) units.'
6. SI units are units used to quantify physical quantities.
They're all based on somewhat random, but standardised measurements. For example, the kg is based on the mass of a block kept somewhere in France.
7. (Original post by oShahpo)
SI units are units used to quantify physical quantities.
They're all based on somewhat random, but standardised measurements. For example, the kg is based on the mass of a block kept somewhere in France.
what
that's kinda confusing
8. (Original post by sketchymofo2)
more so to do with the fact that there were a lot of older measuring systems and that S.I units just make it standard across the board.
before metric measurements.
well there are metric based systems which aren't SI

e.g. CGS https://en.wikipedia.org/wiki/Centim...ystem_of_units

but you can completely ignore their existence for all current A levels... SI is thankfully all anyone needs to worry about.
9. (Original post by CorpusLuteum)
what
that's kinda confusing
Suppose you and your friends start collecting stones, and you wanted to quantify how big a stone is, but you didn't have any knowledge of rulers and similar measurement devices, what would you do? Well, I personally would choose a stone of some size and call it a standard stone, and measure the size of other stones in terms of this "standard" stone. So suppose you bring me some stone, and you want to know how big it is, I can tell you for example that it's just about as big as 3 standard stones.

That's exactly what SI units are about, quantifying things. Every time you say for example, this rock is 5 kilograms, what you're really saying is that this rock is 5 times as massive as the "standard" stone. The standard stone is some cylinder *pictured above* kept in France.

The same applies to meters, but instead of mass you're measuring length, as well as all the other units too.
10. (Original post by oShahpo)
Suppose you and your friends start collecting stones, and you wanted to quantify how big a stone is, but you didn't have any knowledge of rulers and similar measurement devices, what would you do? Well, I personally would choose a stone of some size and call it a standard stone, and measure the size of other stones in terms of this "standard" stone. So suppose you bring me some stone, and you want to know how big it is, I can tell you for example that it's just about as big as 3 standard stones.

That's exactly what SI units are about, quantifying things. Every time you say for example, this rock is 5 kilograms, what you're really saying is that this rock is 5 times as massive as the "standard" stone. The standard stone is some cylinder *pictured above* kept in France.

The same applies to meters, but instead of mass you're measuring length, as well as all the other units too.
Aaaaah, thank you.
11. (Original post by CorpusLuteum)
Ah, danke.

Do you guys know what this means:

'Fundamental (base) units.'
Bitte.

Fundamental Units.
It's LIKE expressing the weight of Apples you just bought. They're 3.25 kgs, which can be expressed as 3250 gms. Same meaning, different units. So fundamental units, are well, units that other units can be measured by.

Now, ignore the Apple example because it's wrong. Yes it is, I just mentioned it so you understand units better. Sorry if I made things worse

So, take Pressure. Pressure is Force/Area.

SI of Force is? Newtons = N
SI of Area is? Sq. Metres = m2 (That 2 is supposed to be a superscript)

Thus, the SI unit of Pressure is the the SI unit of Force/SI unit of Area = N/m2

But, Newtons is not a fundamental unit.
Force = Mass * Acceleration
Thus, N = kg * m/s2 (Again, that's sq. metre but I can't find the damn superscript.)

Now, N = kgm/s2
And P = N/A
= (kgm/s2) / m2
Which cancels out one metre, and gives you the SI unit of Pressure as kg/ms2
12. (Original post by raniafern)
Bitte.

Fundamental Units.
It's LIKE expressing the weight of Apples you just bought. They're 3.25 kgs, which can be expressed as 3250 gms. Same meaning, different units. So fundamental units, are well, units that other units can be measured by.

Now, ignore the Apple example because it's wrong. Yes it is, I just mentioned it so you understand units better. Sorry if I made things worse

So, take Pressure. Pressure is Force/Area.

SI of Force is? Newtons = N
SI of Area is? Sq. Metres = m2 (That 2 is supposed to be a superscript)

Thus, the SI unit of Pressure is the the SI unit of Force/SI unit of Area = N/m2
Oh, okay!
I understand now.
13. (Original post by oShahpo)
Suppose you and your friends start collecting stones, and you wanted to quantify how big a stone is, but you didn't have any knowledge of rulers and similar measurement devices, what would you do? Well, I personally would choose a stone of some size and call it a standard stone, and measure the size of other stones in terms of this "standard" stone. So suppose you bring me some stone, and you want to know how big it is, I can tell you for example that it's just about as big as 3 standard stones.

That's exactly what SI units are about, quantifying things. Every time you say for example, this rock is 5 kilograms, what you're really saying is that this rock is 5 times as massive as the "standard" stone. The standard stone is some cylinder *pictured above* kept in France.

The same applies to meters, but instead of mass you're measuring length, as well as all the other units too.
That was better than mine
14. (Original post by CorpusLuteum)
Oh, okay!
I understand now.
Check out the rest
15. (Original post by raniafern)
Check out the rest
Thank youuuu

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