Physical units and vectorA. Physical Quantities and UnitsAny number that is used to describe a

physical phenomenon quantitatively is called a physical quantity.

For example, two physical quantities that describe you are your weight and your height.

1. Fundamenthal Physical QuantitiesSome physical quantities are so fundamental

that we can define them only by describing how to measure them. Two examples are measuring a distance by using a ruler and measuring a time interval by using a stopwatch.

In mechanics, the three basic quantities are length, mass, and time.

All other quantities in mechanics can be expressed in terms of these three.

Other SI standards established by the committee are those for temperature (the kelvin), electric current (the ampere), luminous intensity (the candela), and the amount of substance (the mole).

In our study of mechanics we shall be concerned only with the units of length, mass, and time.

Physical Quantity

name Symbol

length meter m

mass kilogram kg

time second s

temperature kelvin K

electric current ampere A

luminous intensity candela cd

amount of substance

Mole mol

The meter (m) is defined as the distance traveled by light in vacuum during a time of 1/299 792 458 second

The kilogram (kg), is defined as the mass of a specific platinum–iridium alloy cylinder kept at the International Bureau of Weights and Measures at Sèvres, France.

The second (s), is defined as 9 192 631 770 times the period of vibration of radiation from the cesium-133atom.

2. Derived Physical Quantities In other cases we define a physical quantity

by describing how to calculate it from other quantities that we can measure. Thus we might define the average speed of a moving object as the distance traveled (measured with a ruler) divided by the time of travel (measured with a stopwatch).

B. Significant figuresWhen physical quantities are measured, the

measured values are known only to within the limits of the experimental uncertainty.

The value of this uncertainty can depend on various factors, such as the quality of the apparatus, the skill of the experimenter, and the number of measurements performed.

When multiplying several quantities, the number of significant figures in the final answer is the same as the number of significant figures in the least accurate of the quantities being multiplied, where “least accurate” means “having the lowest number of significant figures.” The same rule applies to division.

(5.4 cm)(6.3 cm) = 34 cm2 correct(5.5 cm)(6.4 cm) = 35.2 cm2, mistakeand (5.6 cm)(6.5 cm) = 36 cm2. correct

C. Scalar, vector and vector additionWhen a physical quantity is described by a

single number (magnitude), we call it a scalar quantity.

A vector quantity has both a magnitude and a direction in space.

Calculations that combine scalar quantities use the operations of ordinary arithmetic. For example, 6 kg + 3 kg = 9 kg.

Calculations that combine vector quantities use the operations of cosinus rule.

1. Vector addition

vector addition obeys the commutative law.

2. Vector SubtractionWe can subtract vectors as well as add them. To see how, recall that the vector -A has the

same magnitude as vector A but the opposite direction.

We define the difference A - B of two vectors A and B to be the vector sum of A and -B

D. Vector component

E. Unit vectors and Vector Addition

1. Unit Vectors

2. Vector AdditionWhen two vectors A and B are represented in terms of their components, we can express the vector sum R using unit vectors as follows:

F. Scalar product of vector

G. Vector product of vector

1. Puspa2. Intan3. Anisa