Date post: | 19-Dec-2015 |
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Units come in 3 types:
BASICMass kg M
Length m L
Time s T
Lower case
Capitals
DERIVEDSpeed ms-1
LT-1
Acceleration ms-2
LT-2
Density kgm-3 ML-3And SCIENTISTS: N, J, V, A, Pa, etc
The examiners expect you to be able to explain what the SCIENTIST-UNITS are in BASIC UNITS.
Even worse, you’re expected to use the kg/m/s AND M/L/T ways of writing them!
Don’t worry – they’re easy if you just follow some simple rules.
Rule 1: all derived units can be written as kg/m/s or M/L/T using the formula for the quantity.
e.g. Speed = distance/time ms-1 LT-1
Momentum = mass x velocitykgms-1
MLT-1
Rule 2: JOULES and NEWTONS are the only important scientist-units you’ll need.
NEWTONSNEWTONSRemember your GCSE formulas?
F = m x a
so, Newtons must be kg x ms-2
or M x LT-2
If you can’t work it out, remember it! N are kgms-2
JOULESJOULES
Energy change in J = Force x distance
Work done in J = F x dso…
Joules are kgms-2 x m
which is………..kgm2s-2
or ML2T-2
Try one yourself.
What are Pascals in kg/m/s and M/L/T?
Formula?
Pressure in Pascals = Force/Area
Units?
Pascals will be N/m2
or kgms-2
m2
or kgm-1s-2 or ML-1 T-2
Here’s a tricky one….
Young’s Modulus is STRESS/STRAIN.
Stress is Force/area.
Strain is extension/original length.
What’s the unit of Young’s Modulus?
Hint: units can cancel, just like in formulas.
Try it out and then check your answer with your teacher.
A sig fig tells you how accurate your measurements are.
For example: how accurately could you measure volume with this?
To 1 ml? 0.5 ml? 0.1 ml? 0.01 ml?Probably to ~ 0.2 ml.
Maybe an older teacher couldn’t do as well as this!
That means “to + or – 0.2 ml”.
And that means “+ or – 2/10 ml”.But what if we wrote ~ 0.20 ml?
What would that mean?
It means “+ or – 2/100 ml.
You couldn’t do that with this measuring cylinder – it isn’t accurate enough.
What sort of equipment would you need?Probably a piece of apparatus your school
does not have!
How do you work out the sig figs?How do you work out the sig figs?Rule 1.
If it’s in standard form, count the numbers in the front number.
e.g. 2.37 x 106 3 sig figs
1.5 x 10-4 2 sig figs
2 x 10-23 1 sig fig.
Rule 2. If it’s an ordinary number, count the numbers AFTER ANY ZEROS.
0.00456 3 sig figs
0.02 1 sig fig
0.020 2 sig figs
This counts!
Using standard form makes sig figs much easier.
GOLDEN RULE for exams.GOLDEN RULE for exams.
NEVER QUOTE AN ANSWER TO MORE SIG FIGS THAN IN THE QUESTION.
E.g. 8.31 x 12.3 = ?
Answer: 102 (to 3 sig figs only!)