Estimates, Averages, and

Sig FigsOctober 6, 2015

(2.2 in your books)

Meme Moment

Scientist of the Day

Jonas Salk• Invented polio vaccine• 58000 people used to get

polio in the US each yearo 1/3 of them ended up

partially paralyzedo Iron lung

• Funded by March of Dimes

• Refused to patent invention

• Polio now gone in all but 3 countries

Percent Error• Another way to evaluate accuracy is with percent

error (% err, % E, PE)• In a good experiment, percent error is < 5%

• Sometimes you’ll see “theoretical” or “actual” instead of “true” – this is still OK

• The top half of the equation is often an absolute value I symbol |

Percent Error

• = 2.38%

RangeRange: the spread of data

Math: biggest # - smallest #

In high school/college you might have a different definition, but use this one for now.

8th grade

Anomalous Data• Sometimes you make a data set and one number

looks really weird• This is anomalous data• Anomalous data is useful – it can tell you if your

equipment isn’t working right, or maybe you forgot to control for a variable

• If your averages and percent error are strange, look for anomalous data

Significant Figures• Significant figures = sig figs• How scientists tell each other

how precise a number is.• Sig figs are made up of all the

measured values (increments) + one that we estimateo Like how we measured volume in a

beaker, then estimated in between the smallest marks

• From now on, all answers must be in sig figs!

Sig the Fig

Zeroes in Sig Figs• A number that isn’t zero always counts• Zeroes in the middle always count• Zeroes at the start don’t count• Zeroes at the end don’t count unless there’s a

decimalNumber Significant

Parts# Sig Figs

45357 45357 5

405 405 3

200 200 1

200.00 200.00 5

200.5 200.5 4

0.0045 0.0045 2

10. 10. 2

More Sig Figs!

Number Significant Parts # Sig Figs

2502757 2502757 7

14.058000 14.058000 8

0.000450 0.000450 3

10000 10000 1

67 67 2

0100 0100 1

2.000 2.000 4

2.00200 2.00200 6

3850 3850 3

Adding/Subtracting Sig Figs

• Use the smallest number of decimal places (or tens, hundreds, thousands, etc)

• Round to that number!

Multiplying/Dividing Sig Figs

• Use the smallest number of sig figs• Round to that number!

More Stuff• If you are using a conversion factor like “1000 g/

1 kg” you have unlimited sig figs

• If you are using a counting number like “2 people,” you have unlimited sig figs

Homework• Worksheet! It’s double-sided and due Friday.

• All answers must be in sig figs.

• Worksheet covers 2.1 and 2.2.

Questions• Any questions about the slides?

• Any new questions?