The New SI Units

A student mistakenly writes the unit for force as kg·m/s instead of the correct unit, which is the newton (N). What error did the student make in their understanding of the units for force?

• They forgot to square the time unit.

• They used mass instead of energy.

• They omitted acceleration from the unit.

• They used the unit for momentum instead of force.

Answer explanation

The student incorrectly wrote kg·m/s, which is the unit for momentum. The correct unit for force is newtons (N), derived from kg·m/s², indicating they omitted acceleration from the unit.

Which of the following correctly represents a derived SI unit?

• kilogram

• second

• newton

• meter

Answer explanation

The newton is a derived SI unit for force, defined as kg·m/s². In contrast, kilogram, second, and meter are base SI units, not derived.

A student claims that time can be measured in joules because time is an energy-related concept. Which of the following best explains why this statement is incorrect?

• Joule is the unit of power, not time.

• Joule measures force, not time.

• Joule measures energy, not time.

• Joule is a unit for distance.

Answer explanation

The statement is incorrect because a joule is a unit of energy, not time. Time is measured in seconds, while joules quantify the amount of energy transferred or converted.

Which of the following is an example of an SI base unit?

• joule (J)

• volt (V)

• kilogram (kg)

• hertz (HZ)

Answer explanation

The kilogram (kg) is the only SI base unit among the options. Joule (J), volt (V), and hertz (Hz) are derived units, while the kilogram is the base unit for mass in the International System of Units.

Which of the following prefixes represents the largest multiple of a base unit?

• kilo-

• milli-

• micro-

• centi-

Answer explanation

The prefix 'kilo-' represents a factor of 1,000, making it the largest multiple of a base unit among the options. In contrast, 'milli-' (1/1,000), 'micro-' (1/1,000,000), and 'centi-' (1/100) represent smaller values.

Which of the following pairs is correctly matched between physical quantity and its SI unit?

• energy – volt

• power - joule

• temperature - Kelvin

• force - watt

Answer explanation

The correct pair is temperature - Kelvin, as Kelvin is the SI unit for temperature. The other pairs are mismatched: energy is measured in joules, power in watts, and force in newtons.

The modern definition of the kilogram is based on which fundamental constant?

• the mass of the International Prototype of the Kilogram

• the speed of light

• Planck's constant

• the gravitational constant

Answer explanation

The modern kilogram is defined using Planck's constant, which relates mass to energy. This shift from the physical prototype to a constant allows for greater precision in measurements.

Which SI unit was redefined using the fixed numerical value of the speed of light?

• meter

• second

• kilogram

• ampere

Answer explanation

The meter was redefined in 1983 based on the fixed numerical value of the speed of light in a vacuum, which is exactly 299,792,458 meters per second. This change established a precise link between the meter and the speed of light.

The redefinition of the ampere is now based on the value of which constant?

• Planck’s constant

• Avogadro’s constant

• elementary charge

• Boltzmann constant

Answer explanation

The ampere is now defined in terms of the elementary charge, which is the charge of a single proton. This redefinition aligns with the SI base units and reflects a more fundamental physical constant.

Which of the following explains why the new definitions of SI units are more stable than the old ones?

• They are based on physical objects.

• They rely on invariant constants of nature.

• They change with technological advancements.

• They are easier to measure directly.

Answer explanation

The new SI units are more stable because they rely on invariant constants of nature, such as the speed of light, rather than physical objects that can change or degrade over time.