Physical quantities and measurement techniques

Accurate measurement in physics depends on selecting the correct instrument and using it properly.

Rulers

• Measure length to a resolution (smallest scale division) of 1 mm
• The object must be aligned with the zero mark, with the observer's eye level with the scale to avoid parallax error

Measuring cylinders

• Determine the volume of liquids directly, or the volume of irregular solids by the displacement method
• The reading is taken at the bottom of the meniscus, with the eye level with the scale to avoid parallax error

Time intervals

• Are measured using clocks, stopwatches, or digital timers
• Manual timing introduces human reaction-time error, which is most significant for short intervals
• Timing multiple oscillations and dividing by the number of oscillations produces a more reliable value for a single period

Measuring multiples of a small distance (such as the combined thickness of many identical sheets) and dividing similarly reduces the ruler's resolution error per item.

Extended Extended students must also distinguish scalar from vector quantities.

Scalars

• Have magnitude only
• Examples include distance, speed, time, mass, energy, and temperature

Vectors

• Have both magnitude and direction
• Examples include force, weight, velocity, acceleration, momentum, electric field strength, and gravitational field strength

When two vectors act at right angles, their resultant is found using Pythagoras' theorem for the magnitude and trigonometry for the direction. The graphical method draws both vectors to scale, tip-to-tail, with the resultant as the hypotenuse. A complete answer always states both the magnitude and the direction of the resultant.

At a Glance