**6 The International system of units Unit Prefixes – Problems and Solutions**

__The International System of Units__

1. Which of the following units is not a unit of length

A. Mil

B. Foot

C. Inch

D. Knot

Solution :

Mil, foot, and inch are units of length.

The knot is unit of velocity or speed.

The correct answer is D.

[irp]

2. Determine the unit of the international system of Young’s modulus

A. kg.m/s^{2}

B. kg/m.s^{2}

C. m/kg.s^{2}

D. kg.s^{2}/m

__Known :__

Unit of force = kg m/s^{2}

Unit of area = m^{2}

Unit of length = m

__Wanted:__ The unit of the international system of Young‘s modulus.

__Solution :__

The correct answer is B.

[irp]

3. Quantity has unit kg.m^{-1}.s^{-2} is…

A. Pressure

B. Energy

C. Momentum

D. Acceleration

__Solution :__

Pressure = Force / Area

The international unit of force is kg m/s^{2}

The international unit of area is m^{2}

The international unit of pressure = kg /m s^{2 }= kg m^{-1} s^{-2}

Energy = Work

Work = Force x Displacement

The international unit of force is kilogram meter per second squared (kg m/s^{2})

The international unit of displacement is a meter (m)

So the international unit of energy = unit of work = kg m^{2}/s^{2} = kg m^{2} s^{-2}

Momentum = Mass x velocity

The international unit of mass is kg

The international unit of velocity is m/s

So the international unit of momentum is kg m/s = kg m s^{-1}

Acceleration = velocity / time interval

The international unit of velocity is meter/second (m/s)

The international unit of time is second (s)

So the international unit of acceleration is m/s : s = m/s x 1/s = m/s^{2} = m s^{-2}

The correct answer is A.

[irp]

4. The international unit of power is…

A. kg m^{2} s^{-2}

B. kg m^{2} s^{-3}

C. g cm^{2} s^{-1}

D. g cm s^{-3}

Solution :

Power = Work / time

The international unit of work is kilogram meter squared per second squared (kg m^{2}/s^{2})

The international unit of time is second (s)

So the unit of power is kg m^{2}/s^{3} = kg m^{2} s^{-3}

The correct answer is B.

__Unit Prefixes__

5. The order of prefix for units from big to small that correct is…

A. Mega, tera, nano, pico

B. Giga, Mega, Micro, nano

C. Pico, Nano, Mega, Tera

D. Mega, Tera, Micro, Pico

Solution :

The correct answer is B.

[irp]

6. A micro century is same as ….

A. 1 hour

B. 90 minutes

C. 52 minutes

D. 4000 seconds

__Known :__

1 day = 24 hours

1 hour = 60 minutes

1 minutes = 60 seconds

1 day = 24 x 60 minutes = 1440 minutes

1 day= 1440 x 60 seconds = 86,400 seconds

__Wanted :__ 1 micro century = ….. hours = ….. minutes = ….. seconds

__Solution :__

1 micro = 10^{-6}

1 micro century = 10^{-6} century = 0.000,001 century

1 century = 100 years = 100 x 365 years= 36,500 years

1 micro century = 0.000,001 x 36,500 days = 0.0365 days

1 micro century = 0.0365 x 24 hours = 0.876 hours

1 micro century = 0.876 x 60 minutes = 52.56 minutes

1 micro century = 52.56 x 60 seconds = 3153.6 seconds

The correct answer is C.