Preparation for Labs

Helena Dedic

The World of the Physicist

Physicists only deal with quantities that they can measure.

Physicists try to understand the world by observing phenomena of nature. When they observe phenomena, they confine themselves to characteristics that are objectively measurable and do not deal with those that are subjective, such as emotions.

In the Olympics, a physicist would happily measure the time that it takes a speed skater to cover a 1000 m distance, but would not participate in judging a figure skater's performance, since that judgement includes a subjective element.

For example, imagine that you are at the park and observe that there are swings with children swinging on them. You notice that some swings go back and forth faster than others.

Thinking about this phenomenon a physicist can choose to describe the swings' motion quantitatively, possibly noting the time it takes to go back and forth as well as the properties of the moving object i.e. the length of the swing, the mass of the child sitting on it, how high the swing will go, the age of the child, etc.

In doing this, the physicist has chosen to define the elements of what we call a system by describing its measurable characteristics while ignoring others, e.g. the appearance of the swing. The quantitative characteristics of a system can vary and are thus, called variables.

Physics is an experimental science

Physicists look for patterns. They are interested in seeing how one variable depends on another.

The theories and principles of physics are statements about patterns in the behaviour of systems, e.g., the law of gravity comes from the observation that any object regardless of its characteristics (mass, shape) gains speed in the same way when dropped in a vacuum.

To discover patterns in the behaviour of various physical systems physicists perform experiments. They vary the characteristics of the system, and then observe the changes in its behaviour.

Since any system has a number of variables, physicists systematically investigate the effect of one variable at a time. Otherwise even a simple pattern may be difficult to deduce. Here is the rule for conducting good experiments:

Change only one variable (which is called the independent variable), 
at a time and record the
 impact of its change on 
another variable (which is called the dependent variable), in the system.

For example, physicists want to see how the distance that a ball travels depends on its mass, diameter and amount of time travelled.

Distance, mass, diameter, and time are the variables under consideration. 

The distance travelled, the variable whose changes are observed, 

is called the dependent variable. 

They will have to perform three experiments in this study.

In the first experiment, they would investigate the effect of changing the mass of the ball on the distance travelled keeping the diameter and time period constant.

The mass, the variable which is being systematically changed, 

is called the independent variable. 

The diameter and the amount of time travelled, the variables that are kept constant, 

are called the control variables (these variables are kept constant). 

The distance travelled by the ball is called the dependent variable.

In the second experiment, they would investigate the effect of changing the diameter on the distance travelled keeping the mass and time period constant.

The diameter of the ball would be the independent variable while its mass and the amount of time travelled would be control variables (these variables are kept constant). The distance travelled by the ball is called the dependent variable.

In the third experiment, they would investigate the effect of changing the amount of time for travel on the distance travelled while keeping the mass and the diameter constant.

The amount of time would be the independent variable while mass and diameter of the ball would be control variables (these variables are kept constant). The distance travelled by the ball is called the dependent variable. ( Answer the question 3 ) Independent variable: the variable (one only) which is being systematically changed Dependent variable: the variable (one only) whose changes are observed Control variables: the variables (any number) that are kept constant.

The nature of measurement

Although physicists cannot ever make an exact measurement, they are confident that anyone else who made the same measurement would obtain the same result within a small uncertainty. W hen they make a measurement, x, they indicate its uncertainty as ± Äx. The interval of uncertainty is the interval (x - Äx, x + Äx). For example, acceleration due to gravity on the surface of the earth 'g' is quoted as 9.81 ± 0.01 m/s 2. The uncertainty ± 0.01 m/s2 is an estimate of how accurate the measurement is. The symbolic statement means the following: While the most likely value of 'g' is 9.81 m/s2, it is almost certain that the value of 'g' is within the interval of uncertainty (9.80, 9.82) m/s2.