Measurement In Physical Science (1963)

measurement in physical science brings to mind complex instruments instruments used with skill to get accurate comparisons accuracy is a prime objective in physical science whether we're determining the thrust a rocket will develop on a test stand the quantity of light received from a distant star or the intensity of radiation released in a nuclear reactor in measuring any physical quantity accuracy depends upon the particular instrument and the skill with which an instrument is used to demonstrate some of the principles involved in making accurate measurements of physical quantities we'll consider the three basic quantities of physical science distance mass and time let's start with distance in this acceleration experiment the boys need to know the distance along this inclined plane the boys use a rather crude instrument a cloth measuring tape no matter how skillfully they use the tape it will be difficult for them to get an accurate measurement the tape sags when the pull it sufficiently tight to make it follow a straight line the soft material stretches slightly the crudeness of the instrument limits the accuracy no matter how skillfully the measurement is made by substituting a steel measuring tape the boys can get a more accurate measurement we can show other ways in which the skills of an observer and the quality of an instrument complement one another in this experiment involving intensity of nuclear radiation a certain degree of skill is required to measure distance even with a comparatively simple instrument this meter stick we'll measure the distance from the probe of the Geer counter to the piece of metal shielding that has been placed in front of a radioactive Source notice that the shield is not lined up with the end of the stick the end is worn so the first division is short this source of error is eliminated by measuring between points farther up the scale another potential source of error is the thickness of the stick the scale is separated from the probe so that our reading of the scale changes as we change our angle of view this source of error is called Parallax it is inherent in the way we see things in perspective to reduce Parallax a skilled Observer will turn the stick bringing the scale markings closer to the probe even now however there is a small space between the scale and the rounded end of the probe if the experiment requires greater accuracy we can select another instrument more appropriate for the job a mirror scale when the Observer is in the position where the mirror image is lined up exactly with the end of the probe his line of vision is perpendicular to the scale giving a more accurate reading our skill in using an instrument depends on knowing which instrument is most appropriate for a given job for example suppose we want to measure the thickness of the metal shield the metal is so thin we cannot use the smallest units on the meter stick because we cannot distinguish distances much smaller than a millimeter so in this case a more accurate instrument to use is the micrometer caliper to tell when the caliper is in contact with both sides of the shield we depend on our sense of touch we can measure the thickness of The Shield to within a 100th of a millimeter with the micrometer have you ever thought of a microscope as a measuring instrument with a microscope our sense of sight is extended and we can measure living cells that are only several thousands of a millimeter in diameter again it is the design and quality of an instrument combined with the skill in using it that results in an accurate measurement of distance in measuring mass the same principles apply in this impact experiment consider the mass of one of the balls how can we get an accurate determination of its mass first we need a set of standard metric masses for comparison a direct visual comparison of masses is not likely to be very accurate we could make an indirect comparison with the aid of an instrument such as a spring balance the balance is calibrated to show weight of the standard masses keep in mind that there is a difference between mass and weight when we weigh an unknown Mass we are comparing the weight of the standard with the weight of the unknown mass mass of an object at rest is always the same the weight is not weight depends on gravity and gravity of course depends upon the mass of the earth and the distance from Center at the Equator this distance is greater than at the poles due to the Earth's equatorial bulge so if an object at one of the poles weighs 100 g at the Equator the same object will weigh about 992 G if an object is sent far enough away from the earth it may approach the state called weightlessness experienced by men in certain kinds of flight on the moon an object will weigh less than on the earth due to the moon's lower gravitational attraction since weight depends upon position if we wish to make an accurate comparison between masses we should weigh them at the same place to do this we select an instrument appropriate for the job the beam balance with the beam balance the standard masses are used in the actual comparison so the unknown mass is being compared directly with the standard masses when the mass is Balan we know that the quantity of Mass on each side is the same this is the principle of the beam balance beam balances like other measuring instruments are available with differing degrees of accuracy this one can measure a mass of 1,000 lb to within 1 2,000 of a pound the micro balance being used here is accurate to within 1 millionth of a gram the quality of the instrument and the skill with which it is used determine the accuracy with which the Mass is measured to show how the principles we have observed apply to the measurement of time we'll use this stop clock which can measure to within 100th of a second for our example we'll consider the vibration of an inertia balance we'll measure the interval of time it takes for the mass to swing past the vertical rod and then back again we'll start the clock when the mass is exactly opposite the rod and stop it when it Returns the clock indicates the interval to the nearest hundredth of a second but is this accurate watch as this student resets the clock and measures the interval once again see if you notice any sources of error what about the way he looks at the balance is Parallax a factor here what about his reaction time between seeing the mass pass in front of the indicator and pressing down on the start button aside from the variation between this reading and the last one our general knowledge tells us that reaction time interferes with the accuracy of the measurement realizing the limitations of his senses a skilled Observer May modify his instrument to compensate for this in this case by using a photoelectric device a projector will provide a beam of Light which will be received by a photosensitive cell the card will cut through the beam of light the edge of the card lined up with the center of the mass cuts through the beam of light and activates the photo cell the cell automatically starts and then stops the clock to which it is connected now the time intervals will be measured with greater accuracy this is another way of compensating for limitations of our senses a clock wired to a photo cell is just one of many instruments skillful observers have devised for getting accurate measurements of time to measure intervals between heartbeats scientists have developed the electrocardiograph this device compares the time between heartbeats with the time it takes the paper to move past the pen the oscilloscope is another device used to measure time the wave pattern shown on this screen indicates the intervals of time between fuel ignition in the cylinders of the engine time remember is one of the basic quantities of physical science the principles we have seen for measuring these basic quantities of distance mass and time apply equally well to other physical measurements whether whether our measurement problem requires a highly Specialized or a routine procedure the design and quality of our instruments must be appropriate to the job and we must use them skillfully to get accurate measurements of physical quantities