# Difference between Si units and Base Si?

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#1
What is the difference between these two?
When we do calculations do we always convert into si units?

Slightly irrelevant to si units but when we drop something off a cliff, do we refer u=0 or v=0. I never understand which one is which, is u/v negative if we drop something off a cliff?

Confused with a few concepts any help would be grateful.

Thanks
0
6 years ago
#2
(Original post by Super199)
What is the difference between these two?
When we do calculations do we always convert into si units?
SI = Le Système International d'unités. This is a global metric standard, widely used but not exclusively. Although for GCSE and A-levels SI is mandatory.

There are seven base Si units which can all be traced back to reliable, repeatable and reproducable reference quantities. i.e. the definition of accepted scientific rigour. These are:

ampere - current (A)
candela - luminous instensity (cd)
kelvin - thermodynamic emperature (K)
kilogram - mass (kg)
meter - length (m)
mole - amount of substance (mol)
second - time (s)

ALL other SI quantities are derived from equations and defined from the seven above.

Note that derived SI quantities always have more than one component.
For example:

area = length x width = m x m = m2
volume = length x width x height = m x m x m = m3
speed = distance / time = m/s = m.s-1
acceleration = speed / time = m/s/s = ms-2
force = mass x acceleration = kg.m.s-2

etc. etc.

(Original post by Super199)
Slightly irrelevant to si units but when we drop something off a cliff, do we refer u=0 or v=0. I never understand which one is which, is u/v negative if we drop something off a cliff?

Confused with a few concepts any help would be grateful.

Thanks
u = initial velocity
v = final velocity

In the case of throwing something off the cliff:

v = u +at

where u = 0, a = 9.81 ms-2

Since velocity is a vector (magnitude and direction), the sign determines the direction component. As long as the reference direction is defined (i.e. against gravity = -ve, with gravity = +ve, or forwards = +ve, bacwards = -ve etc.), and you stick with your convention, the answer will be correct.
1
#3
(Original post by uberteknik)
SI = Le Système International d'unités. This is a global metric standard, widely used but not exclusively. Although for GCSE and A-levels SI is mandatory.

There are seven base Si units which can all be traced back to reliable, repeatable and reproducable reference quantities. i.e. the definition of accepted scientific rigour. These are:

ampere - current (A)
candela - luminous instensity (cd)
kelvin - thermodynamic emperature (K)
kilogram - mass (kg)
meter - length (m)
mole - amount of substance (mol)
second - time (s)

ALL other SI quantities are derived from equations and defined from the seven above.

Note that derived SI quantities always have more than one component.
For example:

area = length x width = m x m = m2
volume = length x width x height = m x m x m = m3
speed = distance / time = m/s = m.s-1
acceleration = speed / time = m/s/s = ms-2
force = mass x acceleration = kg.m.s-2

etc. etc.

u = initial velocity
v = final velocity

In the case of throwing something off the cliff:

v = u +at

where u = 0, a = 9.81 ms-2

Since velocity is a vector (magnitude and direction), the sign determines the direction component. As long as the reference direction is defined (i.e. against gravity = -ve, with gravity = +ve, or forwards = +ve, bacwards = -ve etc.), and you stick with your convention, the answer will be correct.
Thank you very much
I was wondering how is the unit of density formed?
0
6 years ago
#4
(Original post by Super199)
Thank you very much
I was wondering how is the unit of density formed?
density = mass per unit volume

= kg/m3 = kg.m-3
0
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