Laboratory Investigations

Each lab group will submit a single input.

All members of the group will get THE SAME grade UNLESS...

You are observed goofing off or otherwise not contributing to the group effort in which case

your grade will suffer.

In addition to the group grade, many labs will have a quiz the following day.

The quiz will cover only the most important aspects of the lab,

nothing nit-picky.

The grade you get on the lab quiz will be an individual effort and will be added to your group grade.

FormatsThe top of every lab submission should look like this:TITLE: MEASUREMENTS LAB MEMBERS:DATE:LAB TABLE:

EVERY student must be certain of two things:

1. Your full name is on the sheet.

2. You agree with the information submitted.

Good Stuff to Know

In most cases your grade will reflect the accuracy of your conclusions.

To get accurate answers, you must:

1. Get Good Data.

2. Manipulate the data correctly.

3. Accommodate error.

Getting Good Data

You must know what data is required

You must then decide how to obtain it

You must be familiar with your instruments:

capabilities

limitations

Finally, you must know what to do with the data you have obtained.

In very nearly ALL cases, everything you need to know will either be

printed on the lab sheet, or,

told to you prior to the lab.

I leave very little to chance.

Instruments

We will use a collection of instruments of varying sophistication.

rulersstopwatches

electronic timersbalances

graduated cylindersatom smashers, etc.

Most of these instruments you know.

Some you don't know. For those, I will demonstrate.

DO NOT ASSUME your lab partners know the first thing about an instrument.

Watch them, and satisfy yourself that they are "doing it right"

YOUR grade will depend on it.

ERROR

There is error everywhere.

You must remember that, and then know what to do with it.

We will encounter three types of error in our labs:

Instrument Error

A problem with the instrument.

Fixed by knowing what the error is, then removing it mathematically.

Human Error

Not knowing the correct procedure for obtaining data, or,

making a mistake using the data.

Fixed by learning the correct way to obtain and process data,

then doing it that way.

Random Error

No matter how good our equipment is,

no matter how well we run an experiment,

no matter how careful we are with our data and procedures,

we will usually fail to get the correct answer.

The reason is Random Error.

Because of random error, the answers you get will usually be wrong,

HOWEVERThe answers you get will most likely be close to the correct answer. And,

your answers will likely be either greater or smaller than the correct answer – by the same amount.

In Other Words...

If you're measuring speed and the correct answer is 7 m/s, likely answers might be:

6.0 m/s6.5 m/s7.5 m/s8.0 m/s

You are not likely to get 3 m/s or 10 m/s.

In almost every situation, the solution to random error is the same:

Take several measurements (the actual number depends…) and average them.

The average of what you got is 7 m/s.

Interestingly, we never actually got the correct answer, but we wind up with it.

THIS IS IMPORTANTIf you are asked what errors are present,

you must NEVER say Human Error.

Human Error is a mistake.

If you claim Human Error, you are admitting that you knew you made a mistake, and, you should have fixed it.

PERCENT DIFFERENCEThis is a way of evaluating your answer.

It is often OK to get an answer within an acceptable range.

Like target shooting. Hitting the bulls eye is great, but difficult.

Getting all shots close to the bulls eye is OK.

Calculating percent difference is easy. There are two forms. The one you use depends on whether or not there is an actual, correct answer that you should get.

Like calculating the value of acceleration due to gravity, close to the Earth.

There is an accepted answer:

9.81 m/s2

In this case,

• Find the absolute difference between your answer and the accepted value, and

• Divide that by the accepted value.

• Multiply by 100 and add a % sign.

For example: You get 9.7 m/s2

Percent Difference is (9.81 - 9.7)/9.8 = 0.011

x 100 = 1.12

add the % sign = 1.12%

It doesn't matter whether the answer you got was above or below the accepted value,

just how close you were.

The other method is when there is no accepted value.

Such as determining the speed of sound using two different methods.

Find the absolute difference between your two experimental values.

Divide by the average of the two values.

Multiply by 100 and add the % sign.

For example:

You get 345 m/s one way and 340 m/s by another method.

The difference is (345 – 340) = 5

divide 5 by the average (342.5)

5/342.5 = 0.015 => 1.5%

We ignore units of measure since they cancel.