INTRODUCTION TO PHYSICS
Physics is the branch of science that deals with the study of matter in relation to energy. Thus physics explains energy and forces. Physics is the systematic study of the way objects, matter and energy moves, changes and interacts. It is really concerned with how fast things move, when they move and what causes things to move. Those things can be the very large like stars or galaxies or the very small, groups of objects or single objects. It is also about what makes up the fundamental building blocks of the reality we live in.
IMPORTANCE OF PHYSICS
Studying the way things moves and interact in the world is fundamentally useful in all sorts of way. In some cases it is crucial to our survival. Interestingly our own brains have needed to develop an automatic understanding of physics, for example being able to walk or balance requires our brains to make lots of calculations about friction and forces. It plays a role in engineering, medical and surgical research, surveying work.Physics is crucial to virtually all of our modern technology, conveniences and infrastructure from computers to cameras and everyday appliances. Physics is also used in other scientific fields like biology and chemistry. For example: The physics of biology becomes biophysics. Physics of astronomy becomes astro physics and physics of the earth becomes geophysics but physics is useful in everyday situations. Having an awareness of physics can help explain:
· Friction breaks and crashes.
· How water boils or freezes.
· How simple machines work.
· Working out how fast or slow things go.
· Predicting where things go and when they get there.
CLASSIFICATION OF PHYSICS
Typically physics is classified into traditional areas of study. These include:
· Atomic/nuclear- the study of the very small
· Mechanics/Dynamics - how things move
· Electromagenetics- including light and radio waves.
· Thermodynamics - heat and temperature
· Quantum physics - movements of single atoms or particles
· Light/sound (Acoustics)- waves
But often these areas overlap each other. In some cases all use similar principles to describe special circumstances.
MEASUREMENT AND EXPERIMENTATION IN PHYSICS
Introduction
In no subject does measurement play as important a role as in science. Real science cannot exist without measurement. Experiments in Physics involve the measurement of various quantities and a great deal of effort has gone into making these measurements as accurate and reproducible as possible. So certain basic standards of measurements have been established and units agreed upon internationally.
Estimation
If accurate measurement is necessary it is always advisable to estimate. This would help; you to avoid silly mistakes that frequently take place while calculating. For better estimations, specially for large numbers, comparison is easier to make.
Approximation
In measurements approximation also plays an important role. In day to day practice we always use approximation. You must have heard people saying "It is approximately five minutes walk from the church."
Quantitative Versus Qualitative
Most experiments in physics require the observations made to be quantitative rather than qualitative. If observations are only descriptive or qualitative, they are likely to be imprecise and could cause disagreements between experimenters. For example, scientists cannot merely say that an object is large or small. Instead they have to specify its size as a quantity, that is, with a number and using a standard unit such as kilogram. This is called a quantitative observation.
PHYSICAL QUANTITIES
UNITS
Unit is a standard for comparison. In earlier times the measurement of quantity of things was quite arbitrary. In many cases it was related to the dimension of different parts of the human body. These parts were chosen as "units" to measure these quantities. For example, for measuring length, distance between the nose and the fingers or outstretched hand was used as a unit.
British System
In this system the unit of length is foot (F), of mass is the pound (P) and of time is the second (S).
METRIC SYSTEM
A system of measurement which is based upon the powers of ten. Each unit quantity was divided into ten parts and each of these parts into further ten and so on. Multiples of the unit are ten, one hundred, one thousand etc. This was very logical. Once the size of the unit had been determined say, the "meter", submultiples were named decimeter, centimeter, millimeter for one tenth, one hundredth and one thousandth of a meter respectively. Multiples were named as the decameter (x 10), hectometer (x 100) and kilometer (x 1000) etc.
The prefixes used in the system are shown in table below:
In the Metric System there are two commonly used systems of measurement, one based on the Meter, Kilogram and Second (MKS) and the other on the Centimeter, Gram and Second (CGS).
International System of Units
An International System of Units (abbreviated as SI)is based on the metric system of measurement. It helps scientists working in different parts of the world to compare their data (measurements) easily.
v Unit of Length is defined as the length of the path traveled by light in vacuum during a time interval of 1/(2.99792458 X 108) seconds.
v Unit of Mass is defined as "the mass of a particular solid cylinder made of platinum-iridium alloy kept in Paris, known as the International Prototype Kilogram".
v Unit of time, second, is equal to the duration of 9192631770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium –133 atoms.
The following table shows the basic units in the SI system together with their symbols:
Basic Units in the SI System
Usually all small measurements are expressed by using the prefixes - deci, centi, milli, etc. with the units.
For large measurements, we use deca, hecto, kilo etc. as prefixes with the units. The symbol and meaning of each prefix is given below
Units of all other physical quantities can be derived from the basic units and hence are called "derived units". The following table shows the list of various physical quantities, derived formula and corresponding SI Units:
DERIVED UNITS
ACCURACY IN MEASURING
Least Count
It is the smallest reading that can be accurately measured while using an instrument or a device. For example the least count of various measuring devices are listed below:
SIGNIFICANT FIGURES
These express the degree of accuracy of measurements. It is a statement which gives number of digits up to which we are sure about their accuracy. It gives the degree of accuracy or precision made with the instrument. In practical life we depend only on approximate measurements. We ignore small measurements when we are computing large measurements. For example, we may measure the length of a wall as 10 meters and 57 centimeters or 10.57 meters. The actual length of the wall is between 10 meters 57.5 cm and 10 meters 56.5 cm. Now we can say that the length 10.57 meters is correct up to four significant figures.
EXAMPLE 1
(a) 8.88 correct to two significant figures is 8.9, because 8.88 is nearer to 8.9 than to 8.8.
(b) On the other hand 8.82 correct to two significant figures is 8.8. This is because 8.82 is nearer to 8.8 than to 8.9.
EXAMPLE 2
Correct the number 8.5775
(a) up to 2 significant figures = 8.6
(b) up to 3 significant figures = 8.58
(c) up to 4 significant figures = 8.578
EXAMPLE 3
Suppose you are measuring the diameter of a cylinder using a vernier caliper
as 2.38 cm. The accurate value may lie between 2.375 cm and 2.385 cm. In this case figures 2 and 3 are absolutely correct while 8 is reasonably correct. This measurement is said to be accurate up to 3 significant figures.
RULES FOR DETERMINING SIGNIFICANT DIGITS
v All the digits from 1, 2, 3, 4, …., 9 are significant digits.
v Zeros (0s) if they occur between non-zero digits, are significant.
For example, in the number- 325007, 2409, 308, zeros in between the digits are significant.
v The final zeros (0s) of an approximated number when expressed as decimal are significant, e.g.,
(i) 8.70 meters means approximation is to the nearest centimeters (i.e., two decimal places).
(ii) (ii) 5.430 kg means approximation is to the nearest gram (i.e., three decimal places).
v 0s (zeros) which are used only to locate the decimal point are non-significant e.g., 0.007, 0.09, 0.4
Accurate Measurement
No measurement is ever perfectly accurate. Even with high precision instruments some error is inevitable.
There are two main types of errors:
Random errors occur in all measurements. They arise when observers estimate the last figure of the reading on an instrument. These include the noise in the room or the mechanical vibrations in the room. These are called random, because they cannot be predicted. The best way of minimizing the error is to take the average of many readings.
Systematic errors: Such mistakes are not random, but constant. They may cause an experimenter to under estimate or over estimate a reading. Systematic errors may be due to defective equipment - for instance, an incorrectly marked ruler; or they may be due to environmental factors - for instance, the weather conditions on a particular day. While recording time using a stop-watch, your reaction time in starting or stopping the stop-watch will certainly vary at times significantly if you are tired or distracted. At times the variation will be more than a few hundredth of a second.
Percentage Error
While reading the length of a simple pendulum or the length of a resistance wire or while finding the weight of a body using spring balance, mass by a beam balance etc., we are likely to make mistakes. The percentage error can be calculated by using the formula.
Percentage error
For example, if the length of an object (100cm long) is measured as 99.8 cm, then
% error
MEASUREMENT OF LENGTH
Different types of lengths are measured by using different types of instruments. Lengths like the length of cloth or length of a line can be measured by using measuring tape, a metre scale or a foot rule. But these instruments cannot be used to measure the diameter of a metal sphere or a cylinder. To measure the diameter of a cylinder we can use paper strip method and wooden block method:
(i) The correct way to read a ruler is shown in the figure below. The eye must be positioned vertically above the mark to avoid error due to Parallax.
(ii) Paper strip method: Wind a strip of paper closely round the object once and prick the overlapping position with a pin (Shown in figure below).
A method for measuring diameter of a cylinder
Unwind the paper strip and measure the distance between the two pinholes. This measure is the measure of the circumference, since circumference = ( x diameter).
Hence now the diameter can be calculated.
(iii) Wooden block method: Place the sphere or the cylinder between two blocks in contact with a ruler as shown in figure below.
A simple method for measuring diameter of a sphere
Read the distance between the two blocks on the ruler accurately. (The line of sight should be vertical.)
Vernier Calliper
The meter scale enables us to measure the length to the nearest millimeter only. Engineers and scientists need to measure much smaller distances accurately. For this a special type of scale called Vernier scale is used.
The Vernier scale consists of a main scale graduated in centimeters and millimeters. On the Vernier scale 0.9 cm is divided into ten equal parts. The least count or the smallest reading which you can get with the instrument can be calculated as under:
Least count = one main scale (MS) division - one vernier scale (VS) division.
= 1 mm - 0.09 mm
= 0.1 mm
= 0.01 cm
The least count of the vernier
= 0.01 cm
The Vernier calliper consists of a main scale fitted with a jaw at one end. Another jaw, containing the vernier scale, moves over the main scale. When the two jaws are in contact, the zero of the main scale and the zero of the vernier scale should coincide. If both the zeros do not coincide, there will be a positive or negative zero error.
After calculating the least count place the object between the two jaws.
Record the position of zero of the vernier scale on the main scale (3.2 cm in figure below).
Principle of Vernier
You will notice that one of the vernier scale divisions coincides with one of the main scale divisions. (In the illustration, 3rd division on the vernier coincides with a MS division).
Reading of the instrument = MS div + (coinciding VS div x L.C.)
= 3.2 + (3 x 0.01)
= 3.2 + 0.03
= 3.23 cm
Instructions on use of a vernier caliper
· The Vernier caliper is an extremely precise measuring instrument; the reading error is 1/20 mm = 0.05 mm.
· Close the jaws lightly on the object to be measured.
· If you are measuring something with a round cross section, make sure that the axis of the object is perpendicular to the caliper. This is necessary to ensure that you are measuring the full diameter and not merely a chord.
· Ignore the top scale, which is calibrated in inches.
· Use the bottom scale, which is in metric units.
· Notice that there is a fixed scale and a sliding scale.
· The boldface numbers on the fixed scale are centimeters.
· The tick marks on the fixed scale between the boldface numbers are millimeters.