What is physics?

Measurements in physics

Language of physics

PhysicsPhysics

- SI Standards (fundamental units)

- Accuracy and Precision

- Significant Figures

- Uncertainties and Mistakes

- Mathematical Expressions and Validity

What is physics?

- A way of describing the physical world

- Physics is understanding the behavior and structure of matter

- It deals with how and why matter and energy act as they do

6th Century B.C. in the Greek city of Miletus (now in Turkey) a group of men called “physikoi” tried to answer questions about the natural world. Physics comes from the Greek “physis” meaning “nature” and the Latin “physica” meaning natural things

- The word energy comes from the Greek “en”, meaning “in” and “ergon”, meaning “work”. Energy is thus the power to do work. Sounds weird!!

- Energy is the conceptual system for explaining how the universe works and accounting for changes in matter

- Although energy is not a “thing” three ideas about energy are important

1. It is changed from one form to another (transformed) by physical events

2. It cannot be created nor destroyed (conservation)

3. When it is transformed some of it usually goes into heat

-Areas within physics studied this year:

Mechanics – Motion and its causes

Thermodynamics – Heat and temperature

Vibrations and Waves – Periodic motion

Optics – Behavior of light

Electromagnetism – Electricity, magnetism and EM waves

Atomic – Structure of the atom, energy associated with atomic changes

Nuclear – Structure of the nucleus, energy associated with nuclear changes

The Power of Physics

Physics predicts how nature will behave in one situation based on the results of experimental data obtained in another situation.

Mechanics → Newton’s Laws → Rocketry

Electromagnetism → Maxwell’s Equations → Telecommunications

- Investigations in physics generally follow the scientific method

What is physics?

Observations + initial data collection leading to a question, hypothesis formulation and testing, interpret results + revise hypothesis if necessary, state conclusions

Some hypotheses can be tested by making observations.

Others can be tested by building a model and relating it to real-life situations.

- Physics uses models to simplify a physical phenomenon

They explain the most fundamental features of a phenomenon.

Focus is usually on a single object and the things that immediately affect it. This is called the system

Lord Kelvin, who lived in England in the 1800s, was famous for making models.

To model his idea of how light moves through space, he put balls into a bowl of jelly and encouraged people to move the balls around with their hands.

Kelvin’s work to explain the nature of temperature and heat still is used today.

-Models help:

identify relevant variables and a hypothesis worth testing

guide experimental design (controlled experiment)

make predictions for new situations

Today, many scientists use computers to build models.

NASA experiments involving space flight would not be practical without computers.

An airplane simulator enables pilots to practice problem solving with various situations and conditions they might encounter when in the air.

This model will react the way a plane does when it flies.

Measurements in physics

Physics experiments involve the measurement of a variety of quantities.

These measurements should be accurate and reproducible.

The first step in ensuring accuracy and reproducibility is defining the units in which the measurements are made.

Measurements in physics - SI Standards (fundamental units)

Fundamental units: distance – meter (m) time – second (s)

mass - kilogram (kg) temperature - kelvin (K)

current – ampere (A)

luminous Intensity - candela (cd)

Amount of substance – mole (mol) – 6.02 x 1023

Derived units: combinations of fundamental units

speed (v) = distance/time

acceleration (a) = velocity / time

force (F) = mass x acceleration

energy (E) = force x distance

charge (Q) = current x time

units: m/s

units: m/s/s = m/s2

units: kgm/s2 = N (Newton)

units: kgm2/s2 = Nm = J (Joule)

units: As = C (Coulomb)

“It’s better to be roughly right than precisely wrong”

– Allan Greenspan, U.S. Federal Reserve Chairman (retired)

Measurements in physics - Precision and Accuracy

Measurements in physics - Precision

The precision of a series of measurements is an indication of the agreement among repetitive measurements. A “high precision” measurement expresses high confidence that the measurement lies within a narrow range of values.

Precision depends on the instrument used to make the measurement. The precision of a measurement is one half the smallest division of the instrument for analogue and one least significant digit for digital.

The precision of a measurement is effected by random errors that are not constant but cause data to be scattered around a mean value. We say that they occur as statistical deviations from a normalized value.

Random variations can be caused by slight changes in pressure, room temperature, supply voltage, friction or pulling force over a distance. Human interpretation is also a source of random error such as how the instrument scale is read between divisions.

Measurements in physics

- Significant Figures

Significant figures reflect precision. Two students may have calculated the free-fall acceleration due to gravity as 9.625 ms-2 and 9.8 ms-2 respectively. The former is more precise – there are more significant figures – but the latter value is more accurate; it is closer to the correct answer.

General Rules:

2. If there is no decimal point, the rightmost non-zero digit is the least significant.

3. If there is a decimal point, the rightmost digit is the least significant digit, even if it is a zero.

4. All digits between the most significant digit and the least significant digit are significant figures.

1. The leftmost non-zero digit is the most significant figure.

Measurements in physics

- Arithmetic with Significant Figures

When adding or subtracting measured quantities the recorded answer cannot be more precise than the least precise measurement.

Add: 24.686 m + 2.343 m + 3.21 m Answer: 30.24 m

3.21m has the least dp’s

When multiplying or dividing measurements the factor with the least number of significant figures determines the recorded answer

Multiply: 3.22 cm x 2.1 cm Answer: 6.8 cm2

2.1 cm has only 2sf

Note significant digits are only considered when calculating with measurements; there is no uncertainty associated with counting. If you counted the time for ten back and forth swings of a pendulum and you wanted to find the time for one swing, the measured time has the uncertainty but the number of swings does not.

Measurements in physics

- Dealing with uncertainties

If you measured a length of string to be 24.5 cm long this is called a “raw value” because the string is not perfectly straight and there is some interpretation of the scale involved at each end of the ruler. So next you need to estimate the “absolute uncertainty” which could be 0.2 cm.

Because repeated measurement or a more precise measurement might reveal the string’s length to be slightly longer or slightly shorter than the initial value we must express the uncertainty as “plus or minus the absolute uncertainty”.

Therefore: l l = 24.5 cm 0.2 cm = (24.5 0.2) cm

Rules:

1. When adding experimental uncertainties to measured or calculated values, uncertainties should be rounded to one significant figure.

2. The last significant figure in any stated answer should be the same order of magnitude (in the same decimal position) as the uncertainty

Measurements in physics

- Dealing with uncertainties

Suppose you have the following repeated measurement of a measured length. Which measurement is the right one?

Trial Length (cm)

1

2

3

4

5

Mean

value

25.45

25.40

25.50

25.42

25.38

25.43

Clearly the data falls within 0.07 cm of the mean

So you might think length = 25.43 0.07 cm

But… There is uncertainty in the tenths place - it varies from 3 to 5 so back off and write…

length = 25.4 0.1 cm

But… the measurement 25.50 cm looks out of place with the rest of the data so it we ignored it and ran the average again we could get a more precise measurement….

length = 25.41 0.04 cm

Measurements in physics

- Pure Numbers

Lets say you measure the thickness of 100 sheets of paper (26 1mm) and want to know the thickness of a single sheet.

Rule: When you multiply of divide by pure numbers you multiply or divide the absolute uncertainty by the pure number.

So one sheet is (0.26 0.01)mm

Now lets say you measure the diameter of a circular object (D = 4.6 0.2 cm) and want to determine the circumference using c = 2r.

r = D/2 so r r = (2.3 0.1) cm and c = 2r

c c = 2 (rr) = 2 (2.3 0.1) cm

c c = 14.4513 cm 0.6283 cm = (14 1) cm

Measurements in physics

- Combining uncertainties in calculations:

 If you add or subtract, add absolute uncertainty 

If you multiply or divide, find difference between the actual and the maximum and minimum calculated values using the absolute uncertainties.

A rectangular metal plate has length, L = (363)mm and width, W = (18 1)mm what is its circumference?

Cabsolute = 2L + 2W = 2(L+W) = 2(36mm + 18mm) = 108mm

C = 2(L + W) = 2(3mm + 1mm) = 8mm

C = (108 8)mm

What is the area of the plate?

Aabsolute = L x W = 36mm x 18mm = 648 mm2

Amax = Lmax x Wmax = 39mm x 19mm = 741 mm2

Amin = Lmin x Wmin = 33mm x 17mm = 561 mm2

A / 2 = 741 – 648 = 93

A / 2 = 648 – 561 = 87

A = Aabsolute A = (648 93)mm2 = 650 90mm2 = (6.5 0.9)x102 mm2

Measurements in physics

- Accuracy and systematic uncertainty

Systematic uncertainty effects the accuracy of a measurement because it comes from a source of error that is constant throughout a set of measurements. The measurements are consistently shifted in one direction.

A common source of systematic error is not zeroing your measuring instrument correctly so that all data is constantly shifted away from the true value. This can give high precision but poor accuracy.

The accuracy of a measurement is its relation to the true, nominal, or accepted value. It is sometimes expressed as a percentage deviation from the known value. The known or true value is often based upon reproducible measurements.

Instruments might not be accurate. A two-point calibration can be used to check. Does the instrument read zero when it should and does it give a correct value when it is measuring an accepted value

Measurements in physics

- Accuracy and systematic uncertainty

- Percentage Deviation

For example, if a student determines experimentally the acceleration due to gravity is 9.5 m/s2 but the known value is taken as 9.8 m/s2 then …..

The accuracy of a measurement is sometimes expressed as a percentage deviation from the known value. The known or true value is often based upon reproducible measurements.

100%value Actual

value Actual-value YourDifference Percent

3.1% 100%9.8

9.8 - 9.5 Difference Percent

Measurements in physics

- Mistakes

These are a source of error but ARE NOT considered a source of experimental error

Mistakes on the part of the individual such as..

• Misreading scales (using equipment incorrectly)

• Poor arithmetic and computational skills

• Wrongly transferring raw data to the final report

• Using the wrong theory and equations

Language of physics

- Mathematical Expressions and Validity

Physicists use the tools of mathematics to describe measured or predicted relationships between physical quantities in a situation.

A physical equation is a compact statement based on a model of the situation

Like most models, physics equations are only valid if they can be used to make predictions about situations. Physical expressions can be checked for validity a number of ways;

1. through an experiment

2. through dimensional analysis

3. by order of magnitude estimation

Lets say that you are trying to find the mass of an object using, M = V /

Dimensionally: m3 / kg/m3 = m3 m3 / kg = m6 / kg not kg

So the variables are in the incorrect order! (M = V )